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.


Hello @srcole! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-10-28 15:07:53 UTC > also can't see any of the errors listed in issue #28926, so maybe I don't have something set up right.

to see the errors, you would need to comment out the ignore_errors in `setup.cfg` I tested it locally, and it seems this change also fixes:

```
pandas\tests\indexes\test_numeric.py
pandas\tests\indexes\test_category.py
```

So I removed them from `setup.cfg` @srcole can you rebase Thanks for catching that @simonjayhawkins. Yup, I guess I missed that when addressing the merge conflict. Anyway, I removed that line. Thanks @srcole !