CLN: fix mypy errors in pandas\tests\indexes\interval\test_base.py #28926
- [ ] xref #28926
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hi! I based this change on what I saw in #28947. However, when I test this with `mypy pandas/tests/indexes/interval/test_base.py`, I get this error, which I didn't expect:

```
pandas/tests/indexes/interval/test_base.py:16: error: Incompatible types in assignment (expression has type "Optional[Type[IntervalIndex]]", base class "Base" defined the type as "None")
```

I thought the "Optional" that was added would accept the "None." But, additionally, in my local environment, I also can't see any of the errors listed in issue #28926, so maybe I don't have something set up right.

