Fix authz token handling for Keycloak 12

[TheEnbyperor]

Original code throws an error as such on Keycloak 12:

  File "/usr/local/lib/python3.8/site-packages/keycloak/authz.py", line 119, in get_permissions
    decoded_token = self._decode_token(token.split('.')[1])
AttributeError: 'NoneType' object has no attribute 'split'

Seems the required data is now in the access_token attribute.

[codecov-commenter]

Codecov Report

Merging #45 (20ff6b7) into master (08b570d) will decrease coverage by 1.12%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   65.33%   64.20%   -1.13%     
==========================================
  Files          26       26              
  Lines         799      799              
==========================================
- Hits          522      513       -9     
- Misses        277      286       +9     

Impacted Files	Coverage Δ
src/keycloak/authz.py	42.85% <0.00%> (-3.18%)	⬇️
src/keycloak/uma1.py	94.11% <0.00%> (-5.89%)	⬇️
src/keycloak/client.py	78.68% <0.00%> (-4.92%)	⬇️
src/keycloak/uma.py	95.55% <0.00%> (-4.45%)	⬇️
src/keycloak/openid_connect.py	89.09% <0.00%> (-3.64%)	⬇️
src/keycloak/well_known.py	88.88% <0.00%> (+7.40%)	⬆️

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