Enable Ruff PLC (Pylint Convention)

[Avasam]

Ref #13295
https://docs.astral.sh/ruff/rules/#convention-c

The only stub changes are flagged by type-name-incorrect-variance (PLC0105), which enforces the _co/_contra suffix. Which we didn't consistently do. Even sometimes incorrectly suffixed.

import-private-name (PLC2701) is still in preview, so we're not affected yet, but typeshed uses many private imports in its stubs to reference types accurately. I don't think this rule applies to our stubs.
It's unclear whether Ruff should consider disabling it for stubs entirely, but it's mentioned here: astral-sh/ruff#14535

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pyinstrument (https://github.com/joerick/pyinstrument)
- pyinstrument/vendor/decorator.py:295: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G], Callable[..., _G], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
+ pyinstrument/vendor/decorator.py:295: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G_co], Callable[..., _G_co], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
- pyinstrument/vendor/decorator.py:301: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G], Callable[..., _G], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
+ pyinstrument/vendor/decorator.py:301: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G_co], Callable[..., _G_co], tuple[Any, ...], dict[str, Any]], None]")  [assignment]