gnu: python2-dogtail: Switch to Python 3, and add Python 2 variant.

* gnu/packages/python-xyz.scm (python2-dogtail): Rename to python-dogtail.
[name]: Change python2-dogtail to python-dogtail.
[arguments]: Remove #:python.
[propagated-inputs,native-inputs]: Switch Python 2 packages to Python 3
variants.
(python2-dogtail): New variable.