gnu: python-tornado: Rearrange inputs.

* gnu/packages/python.scm (python-tornado)[properties]: Define
python2-variant.
[inputs]: Remove python-certifi, move python-backports-abc here from
native-inputs.
[native-inputs]: Remove python-setuptools.
(python2-tornado): Build with 'strip-python2-variant'.
[native-inputs]: Add python2-setuptools.