fix: type hinting refactoring and small bugfix

[aliev]

For ServerError class the ErrorType value was temporarily_unavailable, it was replaced with server_error value.

Each attribute of error classes now has ErrorType type.

For Literal's using the get_args in order to get the literal's value instead of hard coded strings.

[codecov-commenter]

Codecov Report

Merging #77 (3974b96) into master (48896f2) will increase coverage by 0.02%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   98.78%   98.80%   +0.02%     
==========================================
  Files          14       14              
  Lines         658      669      +11     
  Branches       99      102       +3     
==========================================
+ Hits          650      661      +11     
  Misses          4        4              
  Partials        4        4              

Impacted Files	Coverage Δ
aioauth/grant_type.py	93.02% <ø> (ø)
aioauth/errors.py	100.00% <100.00%> (ø)
aioauth/response_type.py	100.00% <100.00%> (ø)
aioauth/server.py	100.00% <100.00%> (ø)
aioauth/types.py	100.00% <100.00%> (ø)

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