Also moved is_valid_signature() into main package module, for
simplicity.
Also consolidated a handul of in-line pylint disable directives into the
.pylintrc config file.
In addition to the ERC20 codec, also:
Stopped ignoring type errors on 3rd party imports, by including
interface stubs for them;
Removed the unimplemented signature-utils module, which was just a
stand-in when the python project support was first put in place.
https://github.com/0xProject/0x-monorepo/pull/1144
