Pipenv.

- Only install bumpversion on Python 3.6 or later, don't install when
using Python 2.7 (I'm only using it locally and it's not needed when
testing).
- Remove dbus-python and keyring. They were optional requirements for
twine, but dbus-python is deprecated and keyring is now required.
- Don't require a specific Python version, in CI I'm testing with all
supported versions of Python and PyPy and Python 2.7.