deps: move pyfakefs to dependencies (from dev dependencies)

[matthewelwell]

This PR moves pyfakefs from dev dependencies to the main dependencies section.

This is needed because flagsmith-common now includes it's own fixtures which are used in packages that depend on flagsmith-common. Our options are to add pyfakefs as a dependency as we are here, or to add pyfakefs as a dev dependency in all of the repositories which depend upon it.

I have elected for the former as it is cleaner (although not without it's downsides).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.17%. Comparing base (fc2dce3) to head (6552e9c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #79   +/-   ##
=======================================
  Coverage   94.17%   94.17%           
=======================================
  Files          74       74           
  Lines        2147     2147           
=======================================
  Hits         2022     2022           
  Misses        125      125           

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[khvn26]

pyfakefs is already listed as an optional dependency for the test-tools extra.