bug#22808: Add a function to simplify many of the new python2-foo definitions

[Christopher Allan Webber]

Right now we have a lot of these that look very similar:

  (define-public python2-chardet
    (package
      (inherit (package-with-python2
                (strip-python2-variant python-chardet)))
      (native-inputs `(("python2-setuptools" ,python2-setuptools)))))

  (define-public python2-translitcodec
    (package
      (inherit (package-with-python2
                (strip-python2-variant python-translitcodec)))
      (native-inputs `(("python2-setuptools" ,python2-setuptools)))))

  (define-public python2-celery
    (let ((celery (package-with-python2
                   (strip-python2-variant python-celery))))
      (package
        (inherit celery)
        (native-inputs `(("python2-setuptools" ,python2-setuptools)
                         ("python2-unittest2" ,python2-unittest2)
                         ("python2-mock" ,python2-mock)
                         ,@(package-native-inputs celery))))))

Time for an abstraction?