Fix warnings in test session and parser

[bosd]

This commit addresses two warnings that appear during the test session:

1. A FutureWarning in camelot/parsers/hybrid.py related to the downcasting behavior of pandas.DataFrame.replace. The code has been refactored to avoid the use of replace with np.nan and instead uses a more direct method to remove empty rows and columns.

2. A CryptographyDeprecationWarning originating from the pypdf dependency. This warning is suppressed by setting the PYTHONWARNINGS environment variable in the noxfile.py for the test session. This approach is used because filtering the warning via pytest.ini is not feasible due to the way the warning is raised.

Fixes #612

(Sorry for the messy grouping of issues. I am testing / fighting the :robot: Whats matter most is that we are still progressing again in this repo.)