Support infer_variance= parameter for TypeVar (PEP 695)

[JukkaL]

Feature

See this section of PEP for details.

Pitch

This is probably the most significant omission from full PEP 695 support (though this mostly impacts users of older Python versions).

[jonaslb]

this mostly impacts users of older Python versions

Is there a reason that this is expected to be mostly relevant for users of older Python versions? It seems to me to be a very useful addition when implemented. Variance almost always needs to be specified according to the inference rules anyway (and you might need both _co and _contra variants of a type variable instead of only one).

[JelleZijlstra]

@jonaslb because in Python 3.12 and newer, you can use the native generic syntax, which always implicitly infers variance.