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 🤖
Whats matter most is that we are still progressing again in this repo.)