Add devpi docker image

[ryanking13]

This PR adds devpi docker image, which is a private PyPI server that can be used for testing micropip.

devpi is not integrated with micropip tests yet, since micropip does not support alternative registries yet. I will open follow-up PRs.

This PR includes:

• Dockerfile / docker-compose.yml that builds and runs devpi server.
• pytest-docker package that integrates devpi docker with pytest
• Some basic tests that check devpi
• A plugin that adds PyPI JSON API support in devpi

[rth]

Thanks @ryanking13! Some comments are above.

I see the value of having a real server for Python wheels but I think Docker (or installing this server) ideally shouldn't be required to run tests (or at least it should be optional) As far as I know pip doesn't do this for tests. The other alternative is to use this test server, record the response and save them to the file system then mock the response with it. It's a bit more work though.

Maybe @pradyunsg would have some guidance on the best way to test compliance with a server that exposes a PyPI-like JSON API?

[ryanking13]

I see the value of having a real server for Python wheels but I think Docker (or installing this server) ideally shouldn't be required to run tests (or at least it should be optional) As far as I know pip doesn't do this for tests. The other alternative is to use this test server, record the response and save them to the file system then mock the response with it. It's a bit more work though.

Yes, that is a good point. I was also a bit concerned about making docker mandatory to run tests. I like the idea of recording the responses from the test server and mocking the response with it. Basically, I wanted to remove our mock_fetch class, which is very hard to maintain I think.

[ryanking13]

Closing as we added a way to run a mock server in #74.