Add py.typed to ship types

[dobraczka]

Added py.typed to src/pystow in order to fix #67 .
Since v69.0.0 setuptools automatically adds py.typed to the distribution. Since you also add the src directory in your MANIFEST.in this should also ensure this.
I checked whether the proposed fix works locally (it did).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c46485) 66.70% compared to head (bde50fb) 66.70%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #68   +/-   ##
=======================================
  Coverage   66.70%   66.70%           
=======================================
  Files          10       10           
  Lines         949      949           
  Branches      198      198           
=======================================
  Hits          633      633           
  Misses        293      293           
  Partials       23       23           

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[cthoyt]

Thanks @dobraczka! new release coming soon