Trio 0.17.0 updates

[mikenerone]

Fixes #10
Fixes #11

Notes:

1. It appears to have already been the case that (nearly?) all tests fail unless the AMQP_VHOST env var is set to something. I made no attempt to fix that.
2. As a result of 1, it was already the case that tests can't actually succeed via tox (since that env var is not set or passed through in the current tox configuration).
3. Despite tox testing not actual being functional at this time, I still took the liberty of making a couple of updates to tox.ini (updating the envlist and the pytest command, which seemed to not work at all the way it was, as well as updating the renamed allowlist_externals option and setting minversion to ensure the new name is recognized).
4. I only updated the trio dep constraint to >=0.15.0, as I couldn't see any other applicable deprecations/removals since then.
5. I took the liberty of updating import mock to from unittest import mock, as I don't see how that could have worked before, short of manually installing the external mock package.
6. I did not address two existing test failures (one in test_connect.py and one in test_queue.py).
7. There is a new failure in test_consume.py, where pytest-trio claims that the expected error was not raised, but in fact it was. This seems like a bug in pytest-trio (which has also been updated since trio-amqp), but I'm not sure and didn't feel comfortable making the call on the proper way to fix it.

It's very possible that I may have misunderstood the intent in any of these cases - please let me know if there's anything you'd like done differently.

[mikenerone]

I also just noticed there's a commit in v0.2.13 called "Fix tests" that's not in master, although the changes seem mostly obsoleted by changes in master since then, and the remaining ones don't seem related to these particular test failures (which may actually be a result of me not running the tests quite as intended, but I couldn't find any docs, so I'm guessing). I'm not sure what, if anything, from that release should be merged back into master (although probably at least the version bump :P).

[mikenerone]

Closing this because the recent reset to match https://github.com/M-o-a-T/asyncamqp (which is anyio-based) invalidates the trio-interface concerns.