Use a template for git pre-commit hook

Previously, we forced Flake8 to be installed in whatever Python
environment that the user was using. This allows someone to use Flake8
from a virtual environment, e.g., if you use the tox.ini from this
commit, you can do:

    tox -e venv -- flake8 --install-hook git

And that will allow you to use the Python and Flake8 from
./.tox/venv/bin/python. This means that you can avoid installing
Flake8 globally and still have a working commit hook.