Add #ifndef guard around PyBytes_RESIZE

[jakirkham]

Ensure PyBytes_RESIZE is not accidentally redefined by placing it inside an #ifndef/#endif guard block.

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TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.96%. Comparing base (933d901) to head (16546f5).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #732   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          63       63           
  Lines        2738     2738           
=======================================
  Hits         2737     2737           
  Misses          1        1           

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[jakirkham]

Putting in this minor fix to guard against potential compilation errors (none seen yet though)

Runtime usage is unaffected