chore(CI): work around Sphinx and codecov-action issues

[vytas7]

Wrestle CI issues:

• Pin Sphinx to disallow the latest version (3.5.0) that's breaking the viewcode extension: Extension error (sphinx.ext.viewcode) with Sphinx 3.5 sphinx-doc/sphinx#8880.
• Work around codecov issues detecting the commit hash: Issue detecting commit SHA codecov/codecov-action#190.
• Unpin tox since the related issue has been fixed.

[codecov]

Codecov Report

Merging #1862 (15a1f7a) into master (2854011) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1862   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           55        55           
  Lines         5218      5218           
  Branches       842       842           
=========================================
  Hits          5218      5218           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2854011...15a1f7a. Read the comment docs.