Extend to python 3 7

[engeir]

Downgrades numpy, among other libraries, to be able to run with python3.7.

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.82%.

Quality metrics	Before	After	Change
Complexity	3.05 ⭐	3.14 ⭐	0.09 👎
Method Length	83.25 🙂	85.60 🙂	2.35 👎
Working memory	8.39 🙂	8.55 🙂	0.16 👎
Quality	65.56% 🙂	64.74% 🙂	-0.82% 👎

Other metrics	Before	After	Change
Lines	291	291	0

Changed files	Quality Before	Quality After	Quality Change
noxfile.py	77.54% ⭐	76.72% ⭐	-0.82% 👎
tests/test_main.py	46.11% 😞	46.14% 😞	0.03% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
tests/test_main.py	test_main_succeeds	1 ⭐	475 ⛔	13 😞	44.16% 😞	Try splitting into smaller methods. Extract out complex expressions
noxfile.py	activate_virtualenv_in_precommit_hooks	14 🙂	161 😞	9 🙂	50.62% 🙂	Try splitting into smaller methods

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• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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