Fix compatibility with virtualenv 21.0.0

[RRosio]

A few hours ago, virtualenv 21.0.0 was released, and it extracted the Python discovery logic
into a standalone python-discovery package (pypa/virtualenv#3070). As part of this refactoring, propose_interpreters is no longer exported from virtualenv.discovery.builtin, causing hatch to fail with:

Environment `my-env-name` is incompatible: module 'virtualenv.discovery.builtin' has no attribute 'propose_interpreters'

This PR adds a fallback import from python_discovery._discovery (which is installed as a dependency of virtualenv >=21).

The related issue is #2193.

I've tested this locally and it resolved the issue for me.

This is my first contribution to hatch, so I'm happy to adjust the approach if maintainers prefer a different direction!

[flying-sheep]

I don’t think this makes sense, the monkeypatch sets the propose_interpreters attribute on virtualenv.discovery.builtin, but that module no longer has a propose_interpreters function. That function presumably lives in python-discovery now and would need to be patched there.

But as @agriyakhetarpal said, there’s no reason to import this from virtualenv.discovery.builtin anymore, get_interpreter should be directly imported from python-discovery.

After a hotfix, I think it’s best to take this upstream so it never breaks again. This would probably mean to add a filter parameter in python-discovery or so.

[flying-sheep]

python-discovery was updated to accomodate our use case, so no more monkeypatching necessary.

I made a PR based on the new API: #2196

[RRosio]

@flying-sheep thank you!