Fix spurious errors on builtins.open

[hauntsaninja]

Fixes #14796

[github-actions]

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

vision (https://github.com/pytorch/vision) got 1.19x slower (34.1s -> 40.7s)

[ikonst]

Any way to test this? (preferably in a way that clarifies the value and isn't a regression test for #14796)

[hauntsaninja]

I tested manually... I think it's a little hard to write a test, like with #14088

[JukkaL]

I verified manually that this fixes the errors. Thanks!

[AlexWaygood]

I think this change may have introduced a regression. If I run mypy on typeshed's stdlib stubs, it now reports that Module "typing_extensions" has no attribute "LiteralString".

The specific commands I ran were:

1. Checkout 66602fc in my local clone of mypy.
2. Activate a virtual environment with an editable install of mypy pointing to my clone.
3. cd into my local clone of typeshed.
4. Run python tests/mypy_test.py -p3.11 stdlib.

The results of the last command are:

stdlib\builtins.pyi:57: error: Module "typing_extensions" has no attribute "LiteralString"  [attr-defined]
stdlib\builtins.pyi:453: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]
stdlib\builtins.pyi:583: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]


--- exit status 1, 546 files checked ---

These errors cannot be reproduced if I run typeshed's mypy_test.py script using mypy @ b5107a9.