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.

Pipfile

@@ -5,17 +5,10 @@ verify_ssl = true
 
 [dev-packages]
 pre-commit = "*"
-bumpversion = "*"
+bumpversion = {version = "*", markers="python_version >= '3.6'"}
 twine = "*"
-dbus-python = "*"
-keyring = "*"
 template = {editable = true,path = "."}
 
-[packages]
-
-[requires]
-python_version = "3.7"
-
 [scripts]
 lint = "pre-commit run --all-files"
 build = "python setup.py bdist_wheel"