Add __pychache__ to .gitignore
Add __pychache__ to .gitignore

Summary:
When running shell_building, the tree gets littered with __pycache__ files,
hence ignore them.

Reviewed By: wez

Differential Revision: D14664303

fbshipit-source-id: d41837cdfda1ae93c2d9330f00284128b3f7f7d0

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift