Narrow negative TypeIs branches through Any in variant positions

[Dextheking1]

Summary

Fixes #21641.

A negative TypeIs check failed to narrow a generic with Any in a
covariant/contravariant position:

T_co = TypeVar("T_co", covariant=True)

class Source(Generic[T_co]): ...

def is_source(x: object) -> TypeIs[Source[object]]: ...

def f(x: int | Source[Any]) -> None:
    assert not is_source(x)
    reveal_type(x)  # was: int | Source[Any]; now: int

Root cause

restrict_subtype_away() in mypy/subtypes.py uses strict proper subtype
checks, under which Any is only compatible with itself. So
Source[Any] was not recognised as covered by Source[object] purely because
of the Any argument, even though Any is compatible with every type in a
variant position. (The contravariant case already worked because its check goes
through a different path.)

Fix

• Added an allow_any_in_variant_positions option to is_proper_subtype() /
  SubtypeContext: covariant/contravariant type arguments fall back to ordinary
  (Any-compatible) subtype checks, while invariant positions still require exact
  matches. The flag is part of the subtype cache key, so cached results stay
  correct.
• restrict_subtype_away() now retries with this option before giving up, so
  C[Any] is correctly treated as covered by C[object] for variant C.
  Invariant generics (e.g. Box[Any] vs Box[str]) are still not narrowed.

Testing

• New regression test testTypeIsNegativeNarrowCovariantGenericAny in
  test-data/unit/check-typeis.test: fails before, passes after. It covers
  covariant (narrowed), contravariant (still narrowed) and invariant (still kept).
• mypy/test/testsubtypes.py: 26 passed.
• check-typeis, check-narrowing, check-isinstance, check-generics,
  check-protocols, check-inference, check-bound, check-typevar-values,
  check-typeguard: 1269 passed, 6 skipped, 2 xfailed.
• black --check clean; mypy self-check clean.

Note: this is a first-time contributor PR, so some workflow runs may show as
action_required pending maintainer approval.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅