Simplify `cattrs._compat.is_typeddict`

[AlexWaygood]

Following up from https://github.com/python/typing_extensions/issues/230#issuecomment-1587750089! Not sure why the test for generic typeddicts on 3.11 is failing.

This:

• Is simpler
• Is more robust (it doesn't involve importing private implementation details from typing/typing_extensions, which are liable to be changed without warning)
• Doesn't add any additional dependency on typing_extensions except on Python <3.10

[AlexWaygood]

Hmm, the test suite is succeeding on PyPy but the "upload to codecov" bit is failing, and then that leads to all other jobs being cancelled

[codecov-commenter]

Codecov Report

Merging #384 (97774e2) into main (05175ef) will decrease coverage by 0.01%. The diff coverage is 72.72%.

:exclamation: Current head 97774e2 differs from pull request most recent head 019bc63. Consider uploading reports for the commit 019bc63 to get more accurate results

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@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
- Coverage   95.75%   95.75%   -0.01%     
==========================================
  Files          26       26              
  Lines        2122     2121       -1     
==========================================
- Hits         2032     2031       -1     
  Misses         90       90              

Impacted Files	Coverage Δ
src/cattrs/_compat.py	95.45% <72.72%> (-0.48%)	:arrow_down:

... and 1 file with indirect coverage changes

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[Tinche]

Wow Codecov is really being garbage today, sorry about that heh.

[Tinche]

Here's a reduced version of the failing test case:

from typing import TypedDict, Generic, TypeVar
from cattrs._compat import is_typeddict

T = TypeVar("T")

class A(TypedDict, Generic[T], total=True):
    a: T

print(is_typeddict(A[int]))  # False, should be true

[AlexWaygood]

Here's a reduced version of the failing test case:

Ahh, thank you! Should be fixed now -- all tests are now passing for me locally :)

[AlexWaygood]

Also, I should have spotted that behaviour difference between cattrs's version of is_typeddict and the typing(_extensions) one just by looking at the code 🤦

[Tinche]

LGTM, <3 can you just add a line to the changelog? Doesn't have to be very specific, but might help someone down the line if there's a behavior change we're not testing for.

[AlexWaygood]

LGTM, <3 can you just add a line to the changelog? Doesn't have to be very specific, but might help someone down the line if there's a behavior change we're not testing for.

Done! Lmk if that's not quite what you were looking for :)

[Tinche]

Great, thanks!