remove aniso8601, mock, iso8601

[dulmandakh]

Python 3.7 added fromisoformat functionality, so we can remove aniso8601. There are test cases for it, and all are passing..

Removing mock, because the documentation states below, so we don't need it.

mock is now part of the Python standard library, available as [unittest.mock (https://docs.python.org/dev/library/unittest.mock.html) in Python 3.3 onwards.

[dulmandakh]

@syrusakbary please review and merge.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.11%. Comparing base (5fb7b54) to head (dedb1f0). Report is 6 commits behind head on master.

:exclamation: Current head dedb1f0 differs from pull request most recent head 3e588be

Please upload reports for the commit 3e588be to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1548      +/-   ##
==========================================
+ Coverage   96.01%   96.11%   +0.10%     
==========================================
  Files          51       51              
  Lines        1755     1751       -4     
==========================================
- Hits         1685     1683       -2     
+ Misses         70       68       -2     

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