Fix internal mypy error on Windows

[nicoddemus]

When running pre-commit on Windows, I get the error below:

src\_pytest\_py\error.py:95: error: Invalid index type "Optional[int]" for "dict[int, int]"; expected type "int"  [index]
Found 1 error in 1 file (checked 234 source files)

Ignore the error because we do catch the KeyError anyway.

[patchback]

Backport to 8.3.x: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 2f1c143 on top of patchback/backports/8.3.x/2f1c143b17edab8e1525d553e0757a5fc27bb068/pr-13153

Backporting merged PR #13153 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pytest-dev/pytest.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/8.3.x/2f1c143b17edab8e1525d553e0757a5fc27bb068/pr-13153 upstream/8.3.x

4. Now, cherry-pick PR Fix internal mypy error on Windows #13153 contents into that branch:

   $ git cherry-pick -x 2f1c143b17edab8e1525d553e0757a5fc27bb068

   If it'll yell at you with something like fatal: Commit 2f1c143b17edab8e1525d553e0757a5fc27bb068 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 2f1c143b17edab8e1525d553e0757a5fc27bb068

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix internal mypy error on Windows #13153 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/8.3.x/2f1c143b17edab8e1525d553e0757a5fc27bb068/pr-13153

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[nicoddemus]

I guess the change does not need cherry-picking because the typing there is different.