Add file_size to OTA index

[TheJulianJES]

This adds file_size to OTA index generation, as it's required by zigpy since a year or so.

[TheJulianJES]

Wasn't there some recent fix for the "Failed to restore cache entry." issues in some zigpy repo?

• -> Use exact versions in Python matrix workflows#28

We just don't support Python 3.9 anymore, which is still the default.. Removed that from CI in #60.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@          Coverage Diff          @@
##             dev     #59   +/-   ##
=====================================
  Coverage       ?   2.61%           
=====================================
  Files          ?       9           
  Lines          ?     613           
  Branches       ?       0           
=====================================
  Hits           ?      16           
  Misses         ?     597           
  Partials       ?       0           

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