STY: Change if to elif

[j-t-1]

Increases readability.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.36%. Comparing base (5b80cbb) to head (3254996).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pypdf/_encryption.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2968   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files          52       52           
  Lines        8746     8746           
  Branches     1590     1590           
=======================================
  Hits         8428     8428           
  Misses        190      190           
  Partials      128      128           

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