CLN: OrderedDict -> Dict
- xref https://github.com/pandas-dev/pandas/issues/30469
- [x] follow up from https://github.com/pandas-dev/pandas/pull/29212
- [x] passes `black pandas`

As of CPython 3.6 the implementation of dict maintains insertion order.

This became a language feature in python 3.7.

Note in python 3.8 support for reversed(dict) was added - but we don't seem to use that in the use cases i've eliminated.

There is more to do here.