[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

.gitignore

@@ -53,6 +53,8 @@ autoconf/autom4te.cache
 # VS2017 and VSCode config files.
 .vscode
 .vs
+# pythonenv for github Codespaces
+pythonenv*
 # clangd index. (".clangd" is a config file now, thus trailing slash)
 .clangd/
 .cache