[py] Fix type error in mypy

[take0x]

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Description

Because the return type of the add_argument method was not specified, mypy generated a no-untyped-call error.
This error occurs when you call the add_argument method in a typed method.

Motivation and Context

This change will help you avoid unexpected errors that may be generated by typecheckers.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a3b1124) 58.46% compared to head (737e99d) 58.48%.

❗ Current head 737e99d differs from pull request most recent head 6047967. Consider uploading reports for the commit 6047967 to get more accurate results

Files	Patch %	Lines
py/selenium/webdriver/common/options.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #13482      +/-   ##
==========================================
+ Coverage   58.46%   58.48%   +0.01%     
==========================================
  Files          86       86              
  Lines        5270     5270              
  Branches      220      220              
==========================================
+ Hits         3081     3082       +1     
+ Misses       1969     1968       -1     
  Partials      220      220              

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[titusfortner]

Thanks!