guix: python-build-system: Add option "#:use-setuptools?" (default true).

* guix/build-system/python.scm (python-build): New keyword argument
  "#:use-setuptools?", defaulting to #t.
* guix/build/python-build-system.scm (call-setup-py): New positional
  parameter "use-setuptools?". If false, do not use the shim-wrapper
  for addin setuptools. (build, check): accept keyword-
  parameter, and pass to call-setuppy. (install): same; if
  "use-setuptools?" is false, do not use options "--root" and
  "--single-version-externally-managed" for setup.py.
* doc/guix.texi (Build Systems): Document it.