feature: system:user_joined_to_chat event support added

[vladimirgubarik]

fix #494

Added:

• new event support
• tests with factory boy

PR checklist

• I've written good commit message for all commits
• I've split changes into separate commits where it's appropriate
• I've added the description of function to documentation
• I've updated project version in pyproject.toml
• I'll make a release when PR is merged
• I'll bump pybotx in bot-template

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (11dfda6) to head (ac543e9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #495   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          121       122    +1     
  Lines         4249      4281   +32     
  Branches       366       367    +1     
=========================================
+ Hits          4249      4281   +32     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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