explicit copy dataframe in pandas_pdb.py to avoid warnings

[nunziati]

Code of Conduct

Description

Very small modification, only useful to avoid some pandas warnings.

Related issues or pull requests

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./biopandas/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under biopandas/docs/sources/ (if applicable)
• Ran PYTHONPATH='.' pytest ./biopandas -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./biopandas/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./biopandas

[a-r-j]

LGTM! Thanks Giacomo!