Parse Pipfile into a TOMLFile and cache it

        (call clear_pipfile_cache() afterwards if mutating)

def parsed_pipfile(self):
        """Parse Pipfile into a TOMLFile and cache it

        (call clear_pipfile_cache() afterwards if mutating)"""
        contents = self.read_pipfile()
        # use full contents to get around str/bytes 2/3 issues
        cache_key = (self.pipfile_location, contents)
        if cache_key not in _pipfile_cache:
            parsed = self._parse_pipfile(contents)
            _pipfile_cache[cache_key] = parsed
        return _pipfile_cache[cache_key]