Add --pr to open a python/peps PR preview

[hugovk]

Add --pr to open the ReadTheDocs build preview of PRs at https://github.com/python/peps.

For example, for PR python/peps#2440:

$ pep 594 --pr 2440
https://pep-previews--2440.org.readthedocs.build/pep-0594/

[codecov-commenter]

Codecov Report

Merging #1 (40de515) into main (08fe89e) will increase coverage by 3.12%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main       #1      +/-   ##
==========================================
+ Coverage   87.50%   90.62%   +3.12%     
==========================================
  Files           2        2              
  Lines          24       32       +8     
==========================================
+ Hits           21       29       +8     
  Misses          3        3              

Flag	Coverage Δ
GHA_Ubuntu	90.62% <90.90%> (+3.12%)	⬆️
GHA_Windows	90.62% <90.90%> (+3.12%)	⬆️
GHA_macOS	90.62% <90.90%> (+3.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pepotron/__init__.py	85.71% <83.33%> (+2.38%)	⬆️
tests/test_pep.py	100.00% <100.00%> (ø)

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