Compat 21.3

[ahopkins]

Resolves #211

Adds 21.3 support, and also adds a check to throw a runtime warning if you do not set the secret.

[codecov]

Codecov Report

Merging #214 (86f66e6) into main (6b719b1) will decrease coverage by 0.14%.
The diff coverage is 92.85%.

❗ Current head 86f66e6 differs from pull request most recent head d714f28. Consider uploading reports for the commit d714f28 to get more accurate results

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   97.71%   97.57%   -0.15%     
==========================================
  Files          12       13       +1     
  Lines        1050     1070      +20     
  Branches      174      176       +2     
==========================================
+ Hits         1026     1044      +18     
- Misses         15       16       +1     
- Partials        9       10       +1     

Impacted Files	Coverage Δ
sanic_jwt/authentication.py	93.86% <ø> (ø)
sanic_jwt/initialization.py	98.80% <81.81%> (-1.20%)	⬇️
sanic_jwt/__init__.py	100.00% <100.00%> (ø)
sanic_jwt/claim.py	100.00% <100.00%> (ø)
sanic_jwt/configuration.py	99.35% <100.00%> (+<0.01%)	⬆️
sanic_jwt/decorators.py	96.69% <100.00%> (ø)
sanic_jwt/endpoints.py	96.38% <100.00%> (ø)
sanic_jwt/responses.py	100.00% <100.00%> (ø)

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