add .gitignore for __pycache__

[hmoffatt]

Can you please add a .gitignore with an entry to ignore the pycache subdirectory?

I cloned this repo into my homeassistant custom_components as recommended in the README. When I run homeassistant python is creating pycache, which causes the smartthingq checkout to appear as having unstaged files.

[sampsyo]

Hello! It is generally a really good idea to add things like this to your global .gitignore.

No objections to adding it to this repo too, of course—please open a PR if you like!