Drop Python 3.4 and 3.5 compatibility and tests

[horvatha]

Python 3.4 and 3.5 compatibility should be dropped in the new version. End of life of the 3.5 version was 13 Sep 2020.

That way xmltodict needn't use OrderedDict depending on the version. (Actually 3.6 also had end of life on 23 Dec 2021, but its dict is ordered.)

[mpf82]

Insertion-ordered dictionaries are merely a CPython implementation detail in Python 3.6

Starting Python 3.7 this is a guaranteed featured, not just an implementation detail.

That's also the reason for checking 3, 7 instead of 3, 6:

if tuple(map(int, platform.python_version_tuple()[:2])) < (3, 7):

Just my 2 cents, but personally, I don't see a reason to drop support for older Python versions, but then again, it's up to the project's owner.