Add python 3.10 support

[SethMMorton]

Add support for testing and releasing on Python 3.10!

[codecov]

Codecov Report

Merging #50 (5006a75) into master (e9a3af8) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   95.48%   95.51%   +0.02%     
==========================================
  Files           6        6              
  Lines        1042     1047       +5     
==========================================
+ Hits          995     1000       +5     
  Misses         47       47              

Impacted Files	Coverage Δ
src/strings.c	92.19% <0.00%> (+0.02%)	⬆️
src/numbers.c	97.70% <0.00%> (+0.11%)	⬆️

---

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 e9a3af8...5006a75. Read the comment docs.