Reverse order of Mapping imports

[ifigotin]

Currently from collections import Mapping causes the following warning (with Python 3.3 and up):

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working.

The proposed change reverses the order of attempted imports and starts with from collections.abc import Mapping first.

[codecov-io]

Codecov Report

Merging #43 (85c0e59) into master (08b570d) will decrease coverage by 1.37%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   65.33%   63.95%   -1.38%     
==========================================
  Files          26       26              
  Lines         799      799              
==========================================
- Hits          522      511      -11     
- Misses        277      288      +11     

Impacted Files	Coverage Δ
src/keycloak/well_known.py	81.48% <0.00%> (ø)
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/authz.py	42.85% <0.00%> (-3.18%)	⬇️

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