Update docs environment to use Python 3.8+

[mattwthompson]

PR Summary:

After #513, RTD builds are failing. This may be due to the docs environment being pinned to a (now) unsupported Python version. You'd think I would have noticed that after looking at the files off and on for months, but you'd be wrong.

This PR updates that and gives Mambaforge a try (Micromamba isn't supported yet).

PR Checklist

---

• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

Merging #517 (4dd8286) into main (e276bc9) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #517   +/-   ##
=======================================
  Coverage   70.84%   70.84%           
=======================================
  Files          16       16           
  Lines        1650     1650           
=======================================
  Hits         1169     1169           
  Misses        481      481