DictReader can now use basic dicts instead of OrderedDict, as of version
3.7's change to specify that dict maintains keys in insertion order.
This will be more efficient and more pleasant to read at the interactive
prompt.

I also changed a list comprehension to a generator expression inside of
a ``", ".join()`` to avoid the unnecessary list construction.

str.join creates a list internally if the input is not a list or tuple.
Passing a generator expression only adds overhead.

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Follow-up to pythonGH-8014
(cherry picked from commit 7aed052)

Co-authored-by: Éric Araujo <merwok@netwok.org>