ABC: AccountDatabaseAPI's state_root made writable.

AccountDB (from eth/db/account.py) has a setter for the `state_root`
property, and `BaseState` (from eth/vm/state.py) forcibly sets the
state root in its revert() function.

This helps silence `mypy` error:

    eth/vm/state.py:170: error: Property "state_root" defined in "AccountDatabaseAPI" is read-only

eth/abc.py

@@ -1623,6 +1623,16 @@ def state_root(self) -> Hash32:
         """
         ...
 
+    @state_root.setter
+    def state_root(self, value: Hash32) -> None:
+        """
+        Force-set the state root hash.
+        """
+        # See: https://github.com/python/mypy/issues/4165
+        # Since we can't also decorate this with abstract method we want to be
+        # sure that the setter doesn't actually get used as a noop.
+        raise NotImplementedError
+
     @abstractmethod
     def has_root(self, state_root: bytes) -> bool:
         """