Convert requirements in `extras_require` of the form
        `"extra": ["barbazquux; {marker}"]` to
        `"extra:{marker}": ["barbazquux"]`.

def _convert_extras_requirements(self):
        """
        Convert requirements in `extras_require` of the form
        `"extra": ["barbazquux; {marker}"]` to
        `"extra:{marker}": ["barbazquux"]`.
        """
        spec_ext_reqs = getattr(self, 'extras_require', None) or {}
        self._tmp_extras_require = defaultdict(list)
        for section, v in spec_ext_reqs.items():
            # Do not strip empty sections.
            self._tmp_extras_require[section]
            for r in pkg_resources.parse_requirements(v):
                suffix = self._suffix_for(r)
                self._tmp_extras_require[section + suffix].append(r)