Return a parser for parsing requirement lines

def build_parser(line):
    # type: (Text) -> optparse.OptionParser
    """
    Return a parser for parsing requirement lines
    """
    parser = optparse.OptionParser(add_help_option=False)

    option_factories = SUPPORTED_OPTIONS + SUPPORTED_OPTIONS_REQ
    for option_factory in option_factories:
        option = option_factory()
        parser.add_option(option)

    # By default optparse sys.exits on parsing errors. We want to wrap
    # that in our own exception.
    def parser_exit(self, msg):
        # add offending line
        msg = 'Invalid requirement: %s\n%s' % (line, msg)
        raise RequirementsFileParseError(msg)
    # NOTE: mypy disallows assigning to a method
    #       https://github.com/python/mypy/issues/2427
    parser.exit = parser_exit  # type: ignore

    return parser