Add warning for out-of-bounds page range in pdfly cat command

[Zingzy]

PR Description

This PR addresses issue #33 where users attempting to use the pdfly cat command to extract a range of pages outside the number of pages in the PDF document do not receive a warning. The following changes have been made:

• Implemented a warning message: Added logic to check if the specified page range is out of bounds and print a warning to stderr if it is.
• Removed the pytest.skip decorator from the test case: Enabled the test_cat_subset_warn_on_missing_pages test to verify the warning message in stderr.

Testing

• Ran the updated test suite to ensure that the warning message is correctly produced and captured.
• Verified that the test_cat_subset_warn_on_missing_pages test passes, confirming that the warning is printed to stderr when the page range is out of bounds.

[Lucas-C]

Hi @Zingzy

This looks great, thank your for your contribution 👍

Could you just run black pdfly/cat.py and include the changes in this PR? Currently the GitHub Actions pipeline fails because this code formatter has not been applied to this file following your changes: https://github.com/py-pdf/pdfly/actions/runs/11204446524/job/31171421427?pr=58

[Zingzy]

Hello @Lucas-C, I made the necessary changes. 😄

[Zingzy]

Hello @Lucas-C any updates on this?

[Lucas-C]

I merged your MR!

Thank you very much for your contribution @Zingzy 👍