DEPR: min_periods=None behavior for Rolling.count

[mroeschke]

• closes ENH: Change default behavior of rolling.count to be consistent with others #31302
• closes BUG: rolling count on FixedForwardWindowIndexer returns NaN instead of 0 #35579
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Additionally refactors count to take the _apply path to make this op consistent with the others.

[jreback]

make this a FutureWarning. I know its loud but i think we need to.

[jreback]

alt we could do this for a version and then change it.

[mroeschke]

Changed to FutureWarning

[phofl]

This will close #35579 too.

You could use

def test_fixed_forward_indexer_count():
    # GH: 35579
    df = DataFrame({"b": [None, None, None, 7]})
    indexer = FixedForwardWindowIndexer(window_size=2)
    result = df.rolling(window=indexer, min_periods=0).count()
    expected = DataFrame({"b": [0.0, 0.0, 1.0, 1.0]})
    tm.assert_frame_equal(result, expected)

as a test.

[mroeschke]

Thanks @phofl. Added that unit test to this PR

[jreback]

thanks @mroeschke