SG-43906 Packaging for v3.10.3

[stevelittlefish]

Summary

Version bump for the Python 3.7/3.8 cleanup merged in #454, following the historical packaging pattern (e.g. #448 for v3.10.2):

• setup.py: 3.10.2 -> 3.10.3
• shotgun_api3/shotgun.py __version__: 3.10.2 -> 3.10.3
• HISTORY.rst: new v3.10.3 entry

Why

tk-core vendors shotgun_api3 via a real pip install from PyPI (pinned in requirements/*/requirements.txt), not a git checkout - so #454's fix won't reach tk-core until a new version is published. This PR just prepares the version bump; publishing to PyPI still needs to happen through the normal release process (tag + Azure Pipelines) once merged.

Part of epic SG-36195.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.83%. Comparing base (818a453) to head (16d1d75).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #455   +/-   ##
=======================================
  Coverage   78.83%   78.83%           
=======================================
  Files           7        7           
  Lines        1847     1847           
=======================================
  Hits         1456     1456           
  Misses        391      391           

Flag	Coverage Δ
Linux	78.83% <100.00%> (ø)
Python-3.10	78.77% <100.00%> (ø)
Python-3.11	78.77% <100.00%> (ø)
Python-3.13	78.77% <100.00%> (ø)
Python-3.9	78.76% <100.00%> (ø)
Windows	78.89% <100.00%> (ø)
macOS	78.83% <100.00%> (ø)

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