CLN: assorted cleanups

[jbrockmendel]

• closes #xxxx
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[jorisvandenbossche]

Suggested change

	dtype = blocks[0].dtype
	# TODO(EA2D): special case would be unnecessary with 2D EAs
	return isinstance(dtype, np.dtype) # i.e. not ExtensionDtype
	return not self._mgr.any_extension_type

(at this point you know there is only 1 block, so we are not checking it for potentially many columns as was the case before)

[jbrockmendel]

This paves the way for the DTBlock/TDBlock-backed-by-DTA/TDA PR, in which this isinstance check is only slightly tweaked

[jorisvandenbossche]

I suppose that those blocks should then still not be considered as "extension block" for any_extension_type, as the main reason that we use any_extension_type is to know if it has a non-consolidating / non-transposable block (and which would not be the case for DTBlock)

[jbrockmendel]

not worth arguing about, since only one line is in question and that line will get changed in the upcoming PR anyway. will use the proposed edit.

[jbrockmendel]

any other issues here?