BUG: Fix groupby sorting on ordered Categoricals (GH25871)

[kpflugshaupt]

As documented in #25871, groupby() on an ordered Categorical messes up category order when 'observed=True' is specified.
Specifically, group labels will be ordered by first occurrence (as for an unordered Categorical), but grouped aggregation results will retain the Categorical's order.
The fix is a modified subset of #25173, which fixes a related case, but has not been merged yet.

• closes groupby aggregation on ordered Categorial with 'observed=True' breaks order #25871
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @kpflugshaupt! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pandas/tests/groupby/test_categorical.py:

Line 456:36: E241 multiple spaces after ','
Line 458:33: E241 multiple spaces after ','
Line 460:60: W291 trailing whitespace
Line 461:80: E501 line too long (118 > 79 characters)
Line 462:80: E501 line too long (109 > 79 characters)
Line 463:20: E211 whitespace before '('
Line 464:7: E221 multiple spaces before operator
Line 467:80: E501 line too long (80 > 79 characters)
Line 468:1: W293 blank line contains whitespace
Line 469:80: E501 line too long (82 > 79 characters)
Line 470:80: E501 line too long (82 > 79 characters)
Line 472:9: E221 multiple spaces before operator
Line 475:33: W291 trailing whitespace
Line 477:1: W293 blank line contains whitespace

[kpflugshaupt]

Will fix the PEP 8 issues ASAP (sorry -- did not check the test source...)