fix(types): annotate pdb URL fallback list

[Qiaoli-Li-Res]

Summary

• Add an explicit type annotation for the PDB URL fallback list.
• Fix the remaining mypy list item errors in gget_pdb.py.
• No runtime behavior changes.

Validation

• Ran pre-commit run mypy --files gget/gget_pdb.py.

Part of #216.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.44%. Comparing base (5cf607f) to head (db77647).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #255      +/-   ##
==========================================
+ Coverage   56.14%   56.44%   +0.30%     
==========================================
  Files          29       29              
  Lines        9244     9244              
==========================================
+ Hits         5190     5218      +28     
+ Misses       4054     4026      -28     

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[Qiaoli-Li-Res]

The targeted mypy check passes locally with pre-commit run mypy --files gget/gget_pdb.py. The failing pytest cases are all in tests/test_opentargets.py, so they appear unrelated to this gget_pdb.py type-only change.