Fix mypy errors for pandas\tests\series\test_operators.py
part of #28926

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The errors were:
```
pandas/tests/series/test_operators.py:824: error: "Type[Series]" has no attribute "div"
pandas/tests/series/test_operators.py:824: error: "Type[Series]" has no attribute "rdiv"
```
The problem is that these methods are only added at RunTime, so mypy can't see them. I'm not sure if my solution is the best way to silence these errors though.

> The problem is that these methods are only added at RunTime, so mypy can't see them. I'm not sure if my solution is the best way to silence these errors though.

they are added in add_flex_arithmetic_methods which is passed the class as a parameter. They could therefore be specific to a class and so adding them directly in Series (and adding similar in DataFrame separately) seems reasonable to me. @simonjayhawkins the build is green again Thanks @SaturnFromTitan 