Change Authentication hierarchy, fix #61

[lmazuel]

FYI @schaabs This way you can rely on isinstance(..., BasicTokenAuthentication) and you are guaranteed that the following exists:

• credentials.token
• credentials.set_token()
• credentials.signed_session()

[codecov-io]

Codecov Report

Merging #62 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #62      +/-   ##
=========================================
+ Coverage   77.77%   77.8%   +0.03%     
=========================================
  Files         260     260              
  Lines       12696   12680      -16     
=========================================
- Hits         9874    9866       -8     
+ Misses       2822    2814       -8

Impacted Files	Coverage Δ
...lex/auto_rest_complex_test_service/models/shark.py	100% <0%> (ø)	:arrow_up:
...plex/auto_rest_complex_test_service/models/fish.py	100% <0%> (ø)	:arrow_up:
..._rest_validation_test/auto_rest_validation_test.py	64.22% <0%> (+0.25%)	:arrow_up:
...client/auto_rest_parameterized_host_test_client.py	95.45% <0%> (+3.78%)	:arrow_up:
...ervice/auto_rest_required_optional_test_service.py	89.65% <0%> (+4.8%)	:arrow_up:
...est_url_test_service/auto_rest_url_test_service.py	92.85% <0%> (+5.35%)	:arrow_up:
...auto_rest_parameterized_custom_host_test_client.py	92% <0%> (+5.79%)	:arrow_up:

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