Simplify converting units to strings

[eerovaher]

The new code also avoids an upcoming deprecation in astropy 7.0, which closes #499

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (eb9d316) to head (6862639).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   87.28%   87.27%   -0.01%     
==========================================
  Files          22       21       -1     
  Lines        3821     3811      -10     
==========================================
- Hits         3335     3326       -9     
+ Misses        486      485       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[eerovaher]

If you compare the py3-dev job results here with the latest run on master you will see that this pull request has resolved 27 warnings.

[mcara]

Looks good to me and, as an added benefit, it will handle composite units.
Thanks!

[nden]

Thanks!