Add Commit.get_pulls() to Commit.pyi

[atakiya]

Env information

Python version: 3.9.2:1a79785
PyGithub version: 1.55
OS: Windows 10
IDE: PyCharm 2021.1.1

Issue description

IDEs like PyCharm / IntelliJ IDEA report Commit.get_pulls() as unresolved.

What does PR does to resolve the issue

Adds the function to the pyi file

[codecov-commenter]

Codecov Report

Merging #1958 (f4fb555) into master (d1644e3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1958   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files         108      108           
  Lines       11064    11064           
=======================================
  Hits        10937    10937           
  Misses        127      127           

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