MNT: Add py.typed to package

[effigies]

This should allow downstream tools to use our annotations.

Note that not all public interfaces are annotated, so there is the possibility that future annotations will break currently-passing type checks.

With thanks for the nudge from @fepegar.

[codecov]

Codecov Report

Base: 92.16% // Head: 92.16% // No change to project coverage 👍

Coverage data is based on head (08e4256) compared to base (fc49243).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1196   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files          97       97           
  Lines       12332    12332           
  Branches     2534     2534           
=======================================
  Hits        11366    11366           
  Misses        645      645           
  Partials      321      321           

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