BUG: Change IntervalDtype.kind from None to "O"

[jschendel]

• closes BUG: IntervalDtype.kind should be "O" instead of None #30568
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

This is partially blocking #28399 as tests are failing there due to kind being None.

cc @TomAugspurger

[jbrockmendel]

could add a test to the interface tests requiring that dtype.kind be a 1-character str?

[jschendel]

There's an existing test for dtype.kind that allows for it to be None but otherwise checks that it's one of the valid single character string values:

pandas/pandas/tests/extension/base/dtype.py

Lines 17 to 20 in fd7db98

	def test_kind(self, dtype):
	valid = set("biufcmMOSUV")
	if dtype.kind is not None:
	assert dtype.kind in valid

I removed the if block and all the tests are passing for me locally, so I've pushed the change here too. Not sure if there's a specific reason it was written that way and we'd want to allow None though, so can revert if we want to still allow it. Given that None can cause things to break I think we'd want to disallow it though.

[TomAugspurger]

Thanks!