TST: Simplify pathlib PdfReader test

[MasterOdin]

The Path constructor allows a variable amount of arguments to it which it joins together similar to os.path.join works, so it's not necessary to use os.path.join before passing the args to Path.

REPL example:

>>> os.path.join(RESOURCES_PATH, 'crazyones.pdf')
'/Users/mpeveler/code/github/PyPDF2/resources/crazyones.pdf'
>>> Path(os.path.join(RESOURCES_PATH, 'crazyones.pdf'))
PosixPath('/Users/mpeveler/code/github/PyPDF2/resources/crazyones.pdf')
>>> Path(RESOURCES_PATH, 'crazyones.pdf')
PosixPath('/Users/mpeveler/code/github/PyPDF2/resources/crazyones.pdf')

[codecov]

Codecov Report

Merging #1056 (c7bb611) into main (32ab2a3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1056   +/-   ##
=======================================
  Coverage   90.92%   90.92%           
=======================================
  Files          24       24           
  Lines        4508     4508           
  Branches      921      921           
=======================================
  Hits         4099     4099           
  Misses        267      267           
  Partials      142      142           

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[MartinThoma]

Nice! I keep forgetting to use Pathlib 🙈

Thank you for improving PyPDF2 ❤️