Generated api packages: pin a minimum http2py that has client.routes #25

Follow-up from the adversarial review of #24. Packages generated by api_pkg_maker now call api.routes[name], but their setup.cfg lists an unpinned http2py; installed next to an http2py older than #24 they fail on import with AttributeError: ... 'routes'. Write a floor (the version that shipped #24) into install_requires, ideally from http2py.__version__ at generation time.