ENH: Let initialisation from dicts use insertion order for python >= 3.6 (part II)
- [x] xref #19018 and #19830
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR is the second part of the preparation for allowing initialisation using insertion order for python>=3.6. See cross-referenced issues for details.

After this PR all tests will pass independent of dict initialisation rule, and then core/frame.py and core/series.py will be changed in the next PR to use insertion order for python>=3.6.