Update crowd.py

[ChowRex]

Add a new function: group_create
See: https://docs.atlassian.com/atlassian-crowd/5.3.1/REST/#usermanagement/1/group

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 34.37%. Comparing base (1bb6107) to head (f59dc68).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/crowd.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1406      +/-   ##
==========================================
- Coverage   34.37%   34.37%   -0.01%     
==========================================
  Files          46       46              
  Lines        8582     8585       +3     
  Branches     1601     1601              
==========================================
+ Hits         2950     2951       +1     
- Misses       5509     5511       +2     
  Partials      123      123              

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