TYP: Add annotation to assert_frame_equal and assert_series_equal (GH26302)
Incremental typing PR for #26302

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

@WillAyd I looked into `NoReturn` since Python should be >= 3.7 for v1.2.0, but it gave me the mypy error shown below. So I used `None` as I normally do. If you have any hints on how to make `NoReturn` work then LMK

Mypy error when using `NoReturn`: `Implicit return in function which does not return  [misc]` @jreback could you have a look at my comments? Thanks! cc @simonjayhawkins if you can have a look. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. @avinashpancham please ping if you want to pick this up again.