Add py.typed

[eggplants]

I added blank py.typed file into distribution package.

When I using pysub2 mypy raises the error because pysub2 does not PEP 561 typed marker file:

subt/main.py:7:1: error: Skipping analyzing "pysubs2": module is installed, but
missing library stubs or py.typed marker  [import-untyped]
    import pysubs2

pysubs2 type annotations are being checked with mypy --strict, so it can be available.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.13%. Comparing base (dd3a891) to head (15fe96b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   93.13%   93.13%           
=======================================
  Files          36       36           
  Lines        2607     2607           
  Branches      449      449           
=======================================
  Hits         2428     2428           
  Misses        121      121           
  Partials       58       58           

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[eggplants]

py.typed has been added successfully in CI.

https://github.com/tkarabela/pysubs2/actions/runs/10126226229/job/28002628263?pr=98#step:10:196