Point setuptools to file

setuptools_scm 5.0 considers a directory a direct target, rather than a
containing directory, which the build scripts (brokenly) assumed. A
better approach has been known for some time [2], but never implemented.

[1] https://github.com/pypa/setuptools_scm/blob/master/CHANGELOG.rst#v500
[2] pypa/setuptools-scm#131

python/setup.py

@@ -65,7 +65,8 @@ def getversion():
     if not 'SEGYIO_NO_GIT_VER' in os.environ and os.path.isdir(src('../.git')):
         return {
             'use_scm_version': {
-                'relative_to' : src(''),
+                'root': '..',
+                'relative_to' : __file__,
                 # write to ./python
                 'write_to'    : os.path.join(src(''), versionfile),
             }