CLN: OrderedDict -> Dict

[alimcmaster1]

• Follow up from CI: Drop Python 3.5 support #29212

Continue work done in #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: #29923 (comment)

[alimcmaster1]

take

[jbrockmendel]

grepping for OrderedDict i see 29 usages outside of tests; most of those are either in doctests or in pandas.compat.numpy.function

[alimcmaster1]

@jbrockmendel think I have a branch somewhere that finishes this up - will push over the weekend.

Ali