Atomic keys update

[jschlyter]

Rewrite code so we only update self._keys once and inside the writer. Thorough review required!

[codecov-commenter]

Codecov Report

Merging #115 (baacdd1) into main (633118d) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   77.42%   77.46%   +0.04%     
==========================================
  Files          41       41              
  Lines        4328     4327       -1     
  Branches      838      838              
==========================================
+ Hits         3351     3352       +1     
+ Misses        712      711       -1     
+ Partials      265      264       -1     

Impacted Files	Coverage Δ
src/cryptojwt/key_bundle.py	79.17% <100.00%> (-0.03%)	⬇️
src/cryptojwt/jwk/rsa.py	76.37% <0.00%> (+0.78%)	⬆️

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