Add return type to webelement.submit

[adamtheturtle]

Motivation and Context

Without this, mypy in strict mode gives me the error:

Call to untyped function "submit" in typed context

Types of changes

• [x] 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

• [x] 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.
• [x] All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f9ff9b3) 58.48% compared to head (4b749fb) 58.57%. Report is 10 commits behind head on trunk.

:exclamation: Current head 4b749fb differs from pull request most recent head f1c7a38. Consider uploading reports for the commit f1c7a38 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #13490      +/-   ##
==========================================
+ Coverage   58.48%   58.57%   +0.09%     
==========================================
  Files          86       86              
  Lines        5270     5270              
  Branches      220      220              
==========================================
+ Hits         3082     3087       +5     
+ Misses       1968     1963       -5     
  Partials      220      220              

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