Use only ordinary dicts in python >= 3.7

[crdoconnor]

Starting in python 3.7 all dicts are guaranteed to maintain order:

https://mail.python.org/pipermail/python-dev/2017-December/151283.html

OrderedDict is thus unnecessary, although still necessary for any version of python below 3.7.

Drawbacks

• Backwards compatibility: if type(parsed) used to get OrderedDict then upgrades of python may subsequently fail.
• ???

Benefits

• repr of a dict looks a bit nicer
• ???

[opk12]

Backwards compatibility: if type(parsed) used to get OrderedDict then upgrades of python may subsequently fail.

Combining with issue #90 ("Support for type hints"): returning dict for load(..., List[Dict]) and OrderedDict for load(..., Seq(Map())) would be backwards compatible.