Remove default_app_config in robots/ __init__.py

[i-trofimtschuk]

Was only needed for django < 3.2 which are all EOL With newer versions of django it's issueing a RemovedInDjango41Warning

[codecov[bot]]

Codecov Report

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

Comparison is base (4d151bc) 88.99% compared to head (febd693) 88.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
- Coverage   88.99%   88.95%   -0.04%     
==========================================
  Files          13       13              
  Lines         327      326       -1     
  Branches       30       30              
==========================================
- Hits          291      290       -1     
  Misses         26       26              
  Partials       10       10              

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