def _get_provides(self, package: Distribution) -> Dict[str, None]:
        """Extract provided extras from package requirements."""
        provides = {'': None}
        if package.requires:
            for req in package.requires:
                req_marker = (req + ";").split(";")[1]
                if 'extra == ' in req_marker:
                    remove_list = {ord("'"): None, ord('"'): None}
                    provides[req_marker.split('extra == ')[1].translate(remove_list)] = None
        return provides
Extract provided extras from package requirements.
winpython/piptree.py