TypeForm: Enable by default

[davidfstr]

...removing the need to use --enable-incomplete-feature=TypeForm

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REVIEWER NOTES:

• The main objection to merging this PR I can foresee would be an ask to implement one/more of the following issues first:
  • TypeForm: Add covering incremental mode and daemon tests #20125
  • TypeForm: Recognize type variables in a consistent way #20124
  • Any of the 3 older topic-typeform issues, circa 2021 or older

[github-actions]

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

[JelleZijlstra]

hm do we already support 3.15 enough for this to make sense? The test uses typing_extensions anyway so I'm not sure why it needs to pin a version at all.

[davidfstr]

The test uses typing_extensions anyway so I'm not sure why it needs to pin a version at all.

If it's OK for the test to (A) continue to use typing_extensions, then I agree that the --python-version 3.15 doesn't make a lot of sense.

OTOH, if it would be more conventional in tests to (B) use the typing version, then I think --python-version 3.15 becomes necessary.

I'll take a look in the next few days to see what other tests are doing to choose whether to do (A) or (B).