.gitignore: Ignore Python venv files

Since presumably quite a few people will use stuart to build EDK 2,
and since the recommended approach in that case is to use a python
venv, it is much more convenient if these files are ignored by default
in EDK 2, rather than requiring a manual update to .git/info/exclude
each time the repo is cloned, as effectively suggested in the current
How to Build With Stuart instructions on the tianocore wiki.

Signed-off-by: Mike Beaton <[email protected]>

Author: mikebeaton

.gitignore

@@ -5,3 +5,4 @@ Build/
 __pycache__/
 tags/
 .vscode/
+.venv/