Use OrderedDict from typing_extensions

[dmontagu]

Closes #6426

Note that this is the relevant code in typing_extensions:

# 3.7.2+
if hasattr(typing, 'OrderedDict'):
    OrderedDict = typing.OrderedDict
# 3.7.0-3.7.2
else:
    OrderedDict = typing._alias(collections.OrderedDict, (KT, VT))

Given that, I think this is safe to merge. @Kludex I understand where you are coming from with not fixing it but given its so simple I think it's worth just doing it.

Selected Reviewer: @adriangb

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[dmontagu]

Please review