TYP: Subset of "Improved the type stubs in the _libs directory to help with type checking"
The two big parts missing from #43744 are interval.pyi and offsets.pyi as they result in many mypy errors.

I had already made changes to somewhat accommodate for interval.pyi and offsets.pyi (if these files are included, we have now "only" ~100 mypy errors). I kept these changes in this PR.
Hello @twoertwein! 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 2021-12-12 19:24:28 UTC cc @Dr-Irv  > I see that you put some typing in the `pandas/tests/tseries/...` . Have we started typing the test code, and running `mypy` on parts of the test code?

Some tests are typed checked, including those files.

> If not, I don't think that should be included in the PR.

The test changes I made are technically not needed for this PR. They will be needed for `offsets.pyi` from #43744. Happy to remove them and then add them with `offsets.pyi` later.

 > The test changes I made are technically not needed for this PR. They will be needed for `offsets.pyi` from #43744. Happy to remove them and then add them with `offsets.pyi` later.

IMHO, I think the changes to the test files should be left out for now.  That will make the PR smaller.  But I would defer to @simonjayhawkins on this looks good, can you rebase

@simonjayhawkins if any comments. > IMHO, I think the changes to the test files should be left out for now. That will make the PR smaller. But I would defer to @simonjayhawkins on this

The test modules are type checked but the test functions are not (check_untyped_defs is False). Saying that we have several test modules ignored in the mypy config @twoertwein can you merge master thanks @twoertwein very nice