Use uv for dependencies and packaging

[Otto-AA]

Use uv for dependencies and packaging. Fixes #448

I've tried to use the lowest bounds possible for dependencies, so that users of mutmut do not encounter conflicts with their own requirements (but for libcst it is one of the newest versions to support 3.14).

As uv always checks if everything is up-to-date, you only need to run uv run pytest to setup the venv + dependencies and run the tests. Similarly for e.g. uv run mutmut --version.

Building works with uv build. Versioning with uv version (which updates pyproject.toml and uv.lock). Publishing should work with uv publish but I did not test this part.

Added files:

• .python-version (3.10; version that uv uses by default for the virtual environment)
• pyproject.toml
• uv.lock (with the lower bounds of the dependencies, so we do not accidentally break backwards-compatibility while developing)

Removed files:

• MANIFEST.in (covered mostly automatically + source-include in pyproject.toml)
• Makefile (Looked outdated, so I guess you did not use it anyway)
• tox.ini (Looked outdated, so I guess you did not use it anyway)
• requirements.txt & test_requirements.txt (now in pyproject.toml)
• setup.cfg (looked outdated; the pytest testpaths is now in pyproject.toml)
• setup.py (mostly moved to pyproject.toml; however, tag and release utils are gone)

[Otto-AA]

Instead of locking the lowest dependencies we could also switch to newer dependencies locally and use --resolution lowest in the CI. Not sure if we would want to run the CI then with each python version two times, once with lowest and once with highest dependency versions.

[boxed]

Why the move to a src based layout?

[Otto-AA]

Mainly because uv uses the src layout as the default for packaged applications, so it was a little but simpler to follow the docs. Could be changed back to the flat layout, if you prefer that