up requires-python to 3.8

[zacharyburnett]

closes #142

This PR tests Python 3.6 and Python 3.7 updates the minimum Python version required by the package. This will likely change to 3.9 when astropy and numpy drop support for 3.8 (#143)

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[WilliamJamieson]

Why are we bothering testing on 3.6 and 3.7, astropy and numpy haven't supported those versions in several years now?

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -7.97 ⚠️

Comparison is base (f0393e0) 73.13% compared to head (8dea516) 65.17%.

❗ Current head 8dea516 differs from pull request most recent head bc6d6b5. Consider uploading reports for the commit bc6d6b5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   73.13%   65.17%   -7.97%     
==========================================
  Files         101      101              
  Lines        5438     5438              
==========================================
- Hits         3977     3544     -433     
- Misses       1461     1894     +433     

Flag	Coverage Δ
unit	65.17% <ø> (-7.97%)	⬇️

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[zacharyburnett]

I was just running the tests to make sure; it looks like asdf-astropy limits the Python version anyway:
https://github.com/spacetelescope/stdatamodels/actions/runs/4377036134/jobs/7659981886#step:12:31

ERROR: Could not find a version that satisfies the requirement asdf-astropy>=0.3.0 (from versions: 0.1.0, 0.1.1, 0.1.2, 0.2.0, 0.2.1)

so I will up requires-python instead

[braingram]

LGTM.

fixes #142