Improve test coverage

[georgemarshall]

No description provided.

[codecov]

Codecov Report

Merging #80 (18bc3a2) into master (d31005f) will increase coverage by 5.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   92.68%   98.00%   +5.32%     
==========================================
  Files           6        6              
  Lines         410      401       -9     
  Branches       54       54              
==========================================
+ Hits          380      393      +13     
+ Misses         21        3      -18     
+ Partials        9        5       -4     

Impacted Files	Coverage Δ
django_cryptography/fields.py	94.30% <0.00%> (+1.50%)	⬆️
django_cryptography/core/signing.py	100.00% <0.00%> (+9.70%)	⬆️
django_cryptography/typing.py	100.00% <0.00%> (+30.43%)	⬆️

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