Fix bugs in activity.py return types and test collection warning

[jpstroop]

• Change Dict[Never, Never] return types to None for create_favorite_activity and delete_activity_log
• Fix pytest collection warning by refactoring TestResource to be created by a factory function

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1d51869) to head (f01222d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        34           
  Lines         2030      2030           
  Branches       165       165           
=========================================
  Hits          2030      2030           

Files with missing lines	Coverage Δ
fitbit_client/resources/activity.py	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.