CognitoIdp fixes

[mianowski]

• TokenType added to "AuthenticationResult"
• Username corrected in ChalengeParameters

[bblommers]

Hi @mianowski, welcome to Moto!
Would you mind adding some assertions for this? Probably easiest to just add to one of the existing tests here, considering they're quite small changes.

Looks like the linting is failing on our CI as well - you can run make lint to verify this locally.
If black has an issue with the code, make sure that you have the right version (black==19.10b0), as different versions will have different formatting rules.

[codecov-commenter]

Codecov Report

Merging #4518 (333b5dc) into master (9dccebc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4518   +/-   ##
=======================================
  Coverage   95.54%   95.54%           
=======================================
  Files         618      618           
  Lines       65481    65481           
=======================================
  Hits        62562    62562           
  Misses       2919     2919           

Flag	Coverage Δ
servertests	36.11% <ø> (ø)
unittests	95.47% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
moto/cognitoidp/models.py	94.82% <ø> (ø)

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[bblommers]

Thanks for making the changes @mianowski. Happy to merge if we can get the CI to pass

[bblommers]

LGTM - thanks @mianowski!

[github-actions]

This is now part of moto >= 3.1.0.dev5