STYLE use pandas-dev-flaker
- [ ] closes #40826
- [ ] closes #40873
- [ ] 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

----

This means that, just by running `flake8`, you'll be able to get all these custom linting errors alongside the regular `flake8` ones. E.g. if we make `pandas/tests/t.py` as follows:
```python
import pytest

import pandas as pd
from pandas import Categorical


def test_foo():
    cat_0 = pd.Categorical([1])
    with pytest.warns(FutureWarning):
        cat_1 = pd.Categorical(1)
```
then we get:
```console
$ flake8 pandas/tests/t.py 
pandas/tests/t.py:4:1: F401 'pandas.Categorical' imported but unused
pandas/tests/t.py:8:5: F841 local variable 'cat_0' is assigned to but never used
pandas/tests/t.py:8:13: PDF019 found both 'pd.Categorical' and 'Categorical' in the same file
pandas/tests/t.py:9:10: PDF011 found 'pytest.warns' (use 'pandas._testing.assert_produces_warning')
pandas/tests/t.py:10:9: F841 local variable 'cat_1' is assigned to but never used
pandas/tests/t.py:10:17: PDF019 found both 'pd.Categorical' and 'Categorical' in the same file
```

These can then easily be configured using `flake8`'s configuration options, like `# noqa` and `per-file-ignores` in `setup.cfg`.

This also brings down the number of pre-commit checks.

----

I this is something the team wants, and it's decided to keep it as a separate repo, then some outstanding things to do would be:
- transferring it to `pandas-dev`
- sorting out the license (it's currently in my name, and I presume that should change)