Add support for upper case extension file PDFs

[cegea]

Some CAD programs such as Altium Designer generates PDF files with *.SCH instead of *.sch so the FilesOptions class would rise an exception raise ValueError("Input must be a pdf file or a directory.") even though the file exists.

To test it:

@ test_add_watermark_from_options.py: Change INPUT = "tests/fixtures/input.pdf" to INPUT = "tests/fixtures/input.PDF"

[bastienlc]

LGTM, thanks for the PR!