[order_utils.py] is_signature_valid, via Exchange contract (#1216)

First support for signature validation, done via Exchange contract's
isValidSignature() method.

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

@@ -1,10 +1,26 @@
-from typing import Optional, Union
+from typing import Dict, Optional, Union
+
+from web3.utils import datatypes
+
 
 class Web3:
+    class HTTPProvider: ...
+
+    def __init__(self, provider: HTTPProvider) -> None: ...
+
     @staticmethod
     def sha3(
         primitive: Optional[Union[bytes, int, None]] = None,
         text: Optional[str] = None,
         hexstr: Optional[str] = None
     ) -> bytes: ...
+
+    class net:
+        version: str
+        ...
+
+    class eth:
+        @staticmethod
+        def contract(address: str, abi: Dict) -> datatypes.Contract: ...
+        ...
     ...