feat(order_utils.py): ERC20 asset data encoding and decoding

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

python-packages/order_utils/stubs/web3/__init__.pyi

@@ -0,0 +1,10 @@
+from typing import Optional, Union
+
+class Web3:
+    @staticmethod
+    def sha3(
+        primitive: Optional[Union[bytes, int, None]] = None,
+        text: Optional[str] = None,
+        hexstr: Optional[str] = None
+    ) -> bytes: ...
+    ...