Add venv and .venv to .gitignore

[greyli]

Although we use python -m venv env in the contributing guide as the example, we don't need to force the contributor to use env.

[ThiefMaster]

People can use .git/info/exclude to add ignores locally, but I'd like to have .venv on gitignore as well since it's a very common name for virtualenvs, and it saves you one small step after setting up your environment...

[greyli]

I just learned the local gitignore thing, thanks. I normally use venv as the name of the virtual environment.