Add virtualenv locations and .env file to .gitignore

- Add common in-tree locations for virtualenv environments to .gitignore (.env, .venv/, venv/)
- .env is also commonly used to store potentially confidential information such as API keys, so should probably be in here as a matter-of-course safety measure anyway, even though this project doesn't use such files.

.gitignore

@@ -39,3 +39,8 @@ share/
 .coverage*
 # Written by setuptools_scm.
 phonenumber_field/version.py
+# python virtualenv
+.venv/
+venv/
+# python virtualenv or environment file (potential secrets)
+.env