Move requirements in `install_requires` that are using environment
        markers `extras_require`.

def _move_install_requirements_markers(self):
        """
        Move requirements in `install_requires` that are using environment
        markers `extras_require`.
        """

        # divide the install_requires into two sets, simple ones still
        # handled by install_requires and more complex ones handled
        # by extras_require.

        def is_simple_req(req):
            return not req.marker

        spec_inst_reqs = getattr(self, 'install_requires', None) or ()
        inst_reqs = list(pkg_resources.parse_requirements(spec_inst_reqs))
        simple_reqs = filter(is_simple_req, inst_reqs)
        complex_reqs = filterfalse(is_simple_req, inst_reqs)
        self.install_requires = list(map(str, simple_reqs))

        for r in complex_reqs:
            self._tmp_extras_require[':' + str(r.marker)].append(r)
        self.extras_require = dict(
            (k, [str(r) for r in map(self._clean_req, v)])
            for k, v in self._tmp_extras_require.items()
        )