CLN: OrderedDict -> Dict
- Follow up from #29212

Continue work done in https://github.com/pandas-dev/pandas/pull/29923
 
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. ( OrderedDict support this)

Add code check as per comment here: https://github.com/pandas-dev/pandas/pull/29923#issuecomment-559889216