[NFC] Adding pythonenv* to .gitignore

The new feature in GitHub called 'GitHub Codespaces' generates a
pythonenv3.8 directory in the root level of the llvm-project git
checkout. So I am adding that directory to the .gitignore.

See the following for more info:

https://github.com/features/codespaces

Differential Revision: https://reviews.llvm.org/D86846