Add support for Python 3.10

[joshuadavidthomas]

• Bumps pytest to 6.2.5, which added support for 3.10. Relevant PR and tag/release.
• Bumps setup-python action in ci.yml workflow from v1 to v2. 3.10 is not available in v1 of the action; relevant action run.

Passing tests

[codecov]

Codecov Report

Merging #87 (b5900bf) into master (c9a4622) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files           5        5           
  Lines         290      290           
  Branches       31       31           
=======================================
  Hits          288      288           
  Misses          2        2           

Impacted Files	Coverage Δ
watchgod/watcher.py	97.64% <50.00%> (ø)

---

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 c9a4622...b5900bf. Read the comment docs.

[samuelcolvin]

this is great, thank you so much.