Lib3to6

- Add new default integration method: [`lib3to6.Distribution`][href_readme_integration]
- Deprecate old integration method (had issues with package data).
- Fix invalid parsing of install_requires.
- Fix cache invalidation when build config changes.
- Fix type annotations with constants (e.g. `Optional['MyClass']`)
- Fix compat issues with python 3.10
- Disable testing for Python 3.5 and Python 3.6


[href_readme_integration]: https://github.com/mbarkhau/lib3to6#integration-with-setuppy

- Fix type annotations with list arguments

- Fix type annotations with attribute access

- Fixes for Python 3.9 and mypy 0.800

- New 6: per-file opt-in/opt-out using ` lib3to6: disabled`/` lib3to6: enabled`

- New: Lots more documentation.
- New 5: Add detection of invalid imports and point to available backports. Use `install_requires` option to whitelist installed backports.
- New: Checkers produce better error messages.
- New: Colouring of diffs when using `lib3to6` cli command.
- New: Checker for `yield from` syntax on target version doesn't support it
- New: Checker for `` operator when target version doesn't support it
- Fix 3: `--target-version` argument is ignored [gitlab../issues/3](https://gitlab.com/mbarkhau/lib3to6/-/issues/3)
- Fix 4: Remove `from __future__ import X` when the target version doesn't support it.
- Fix 4: Convert Forward Reference Annotations to strings [gitlab../issues/4](https://gitlab.com/mbarkhau/lib3to6/-/issues/4) Thank you [Faidon Liambotis](https://gitlab.com/paravoid) for your help with testing and helping to debug ❤️.
- Fix: Don't apply keyword only args fixer for `--target-version=3.0` or above.