Change Python module build (setup.py / setuptools config) to make it more widely compatible

[johnbartholomew]

This moves us toward a Windows & MSVC-compatible Python module build, and should obsolete #784, and move toward a resolution for #734.

• Change setup.py to compile the code using built in C/C++ extension build system, not using the Makefile.
• Minor fix in python/_jsonnet.c to use Py_ssize_t instead of ssize_t, for compatibility with some MSVC version(s) that don't have ssize_t available.
• Initial GitHub action workflow to build Python wheel packages for Linux and Windows, for various CPython versions. This uses cibuildwheel which - as far as I can judge as an ecosystem outsider - is the current supported/widely used tool for building a set of version-compatible wheel packages.
• Runs python/_jsonnet_test.py for each build as a smoke test.

Things not in this PR:

• Publishing the wheel packages anywhere.
• Automatically running the workflow on relevant events like tagging a release version. The workflow is just set up to allow it to be manually triggered. Note that manually triggered GitHub actions are kind of weird - in particular, there's no way to run them (as far as I know) until the workflow config has been included in the main (HEAD) branch of the repo. So it's possible this workflow could be broken and we probably won't be able to tell for sure until after it's merged to master. However I did try it on my fork (example run: https://github.com/johnbartholomew/jsonnet/actions/runs/8069203253)

Is it tested? Kinda. I don't have a Windows machine to test on, so I'm relying on the GitHub workflow to build & run the Python test.

[johnbartholomew]

Adjusted the workflow configuration to give cibuildwheel a test command to run - this just runs the _jsonnet_test.py script. Currently it's failing for the Windows wheel build. Looks like a problem with relative import paths (perhaps the same or partially the same problem as #867, though that particular report is a little unclear).

Unfortunately it seems all the path handling code only expects / as a directory separator.

   E...EEEE..
  ======================================================================
  ERROR: test_double_import (__main__.JsonnetTests)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "D:\a\jsonnet\jsonnet\python\_jsonnet_test.py", line 172, in test_double_import
      json_str = _jsonnet.evaluate_snippet(
  RuntimeError: RUNTIME ERROR: couldn't open import "trivial.jsonnet": File not found
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:1:11-35	thunk <x>
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:3:1	
  
  
  ======================================================================
  ERROR: test_import_binary_encode (__main__.JsonnetTests)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "D:\a\jsonnet\jsonnet\python\_jsonnet_test.py", line 136, in test_import_binary_encode
      json_str = _jsonnet.evaluate_snippet(
  RuntimeError: RUNTIME ERROR: couldn't open import "binary123.bin": File not found
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:1:1-26	
  
  
  ======================================================================
  ERROR: test_import_binary_sentinel_encode (__main__.JsonnetTests)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "D:\a\jsonnet\jsonnet\python\_jsonnet_test.py", line 145, in test_import_binary_sentinel_encode
      json_str = _jsonnet.evaluate_snippet(
  RuntimeError: RUNTIME ERROR: couldn't open import "binary1230123.bin": File not found
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:1:1-30	
  
  
  ======================================================================
  ERROR: test_import_encode (__main__.JsonnetTests)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "D:\a\jsonnet\jsonnet\python\_jsonnet_test.py", line 118, in test_import_encode
      json_str = _jsonnet.evaluate_snippet(
  RuntimeError: RUNTIME ERROR: couldn't open import "trivial.jsonnet": File not found
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:1:1-25	
  
  
  ======================================================================
  ERROR: test_import_no_eol_encode (__main__.JsonnetTests)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "D:\a\jsonnet\jsonnet\python\_jsonnet_test.py", line 127, in test_import_no_eol_encode
      json_str = _jsonnet.evaluate_snippet(
  RuntimeError: RUNTIME ERROR: couldn't open import "trivial_no_eol.jsonnet": File not found
  	D:\a\jsonnet\jsonnet\python\testdata\test.jsonnet:1:1-32	
  
  
  ----------------------------------------------------------------------
  Ran 10 tests in 0.625s
  
  FAILED (errors=5)
Error: Command python D:\a\jsonnet\jsonnet/python/_jsonnet_test.py

(Build action log: https://github.com/johnbartholomew/jsonnet/actions/runs/8087521792/job/22099656631)

[johnbartholomew]

I did the naive thing and just added an #if switch on _WIN32 to additionally look for backslash as a directory separator. We can consider switching to the C++17 lib, but I'd prefer to start with a smaller change like this that should be no worse than what we have right now.

In GitHub actions the Python test suite now passes on Windows: https://github.com/johnbartholomew/jsonnet/actions/runs/8098334188/job/22131487672

  + pip install 'C:\Users\runneradmin\AppData\Local\Temp\cibw-run-e8_jxk3n\cp312-win_amd64\repaired_wheel\jsonnet-0.20.0-cp312-cp312-win_amd64.whl'
  Processing c:\users\runneradmin\appdata\local\temp\cibw-run-e8_jxk3n\cp312-win_amd64\repaired_wheel\jsonnet-0.20.0-cp312-cp312-win_amd64.whl
  Installing collected packages: jsonnet
  Successfully installed jsonnet-0.20.0
  + python D:\a\jsonnet\jsonnet/python/_jsonnet_test.py
  ..........
  ----------------------------------------------------------------------
  Ran 10 tests in 0.505s
  OK