Fix requirements for Python 3.7 because of https://github.com/ericvsmith/dataclasses/pull/161 (#1730)
Fix requirements for Python 3.7 because of https://github.com/ericvsmith/dataclasses/pull/161 (#1730)

* Build `requirements.txt` on Python 3.7 to fix an issue with installing dataclasses. 
* Closes #1729 

I tested that this works in Python 3.6, 3.7, and 3.8. A useful project would be to add installing deps to Github Actions. 
I'll subscribe to https://github.com/ericvsmith/dataclasses/pull/161 since this is IMO a bug in dataclasses. 
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