Ignore pyenv config file.
Among other possibilities, [pyenv](https://github.com/pyenv/pyenv)
allows directory-scoped Python installation choices, which can be handy
when developing on the same system for projects with different Python
version requirements. For this, it uses a "hidden" `.python-version`.
Such files should not clutter up `git status` messages or be easily
accidentally included in a commit. This change adds `.python-version`
files to `.gitignore`
Change-Id: I88bbe813966282755097da674cb53d1f88a8e031