tools: add pythonenv to .gitignore

GitHub Codespaces installs a copy of Python 3.8 in the repository.

PR-URL: #35389
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.gitignore

@@ -39,6 +39,8 @@
 /*.msi
 /*.wixpdb
 /*.qlog
+# For GitHub Codespaces
+/pythonenv*
 
 # === Rules for artifacts of `./configure` ===
 /icu_config.gypi