Removed python-xdist development dependency due to it not being used

[caronc]

Description:

Related issue (if applicable): NixOS/34721

Thanks to the amazing team over on NixOS, they pointed out that while Apprise references the python-xdist package, it does not actually use it. There is confusion as those who choose to use it do not get the desired/expected test results.

Removing it from the Apprise library for consistency.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.37%. Comparing base (19c0d8a) to head (4b385ea).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1223   +/-   ##
=======================================
  Coverage   99.37%   99.37%           
=======================================
  Files         148      148           
  Lines       20675    20675           
  Branches     3679     3679           
=======================================
  Hits        20545    20545           
  Misses        121      121           
  Partials        9        9           

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