add Python 3.10 rc

[jschlyter]

It's time to start testing with Python 3.10 pending the release in October

[codecov-commenter]

Codecov Report

Merging #87 (6893f1e) into main (ca85903) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   77.69%   77.71%   +0.02%     
==========================================
  Files          41       41              
  Lines        4312     4316       +4     
  Branches      834      835       +1     
==========================================
+ Hits         3350     3354       +4     
- Misses        700      701       +1     
+ Partials      262      261       -1     

Impacted Files	Coverage Δ
src/cryptojwt/key_issuer.py	81.13% <ø> (+0.62%)	⬆️
src/cryptojwt/key_jar.py	76.31% <ø> (ø)
src/cryptojwt/serialize/item.py	100.00% <ø> (ø)
src/cryptojwt/tools/keyconv.py	42.50% <ø> (ø)
src/cryptojwt/tools/keygen.py	0.00% <ø> (ø)
src/cryptojwt/jwk/rsa.py	76.51% <0.00%> (-0.32%)	⬇️
src/cryptojwt/tools/jwtpeek.py	0.00% <0.00%> (ø)
src/cryptojwt/jwe/jwe.py	87.50% <0.00%> (+0.10%)	⬆️
src/cryptojwt/jwt.py	76.66% <0.00%> (+0.11%)	⬆️

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