Adding new file rule generated by the built-in venv module in Python 3

As described on https://docs.python.org/dev/library/venv.html#creating-virtual-environments

Global/VirtualEnv.gitignore

@@ -5,3 +5,4 @@
 [Ii]nclude
 [Ll]ib
 [Ss]cripts
+pyvenv.cfg