eth/vm/state: expect Hash32 (not bytes) in __init__ of BaseState.

Again, 99.99% cargo-culting. This just silences a `mypy` error:

    eth/vm/state.py:57: error: Argument 2 to "AccountDatabaseAPI" has incompatible type "bytes"; expected "Hash32"

A valid state root will definitely be a 32-byte-long hash. Question
is if this is the right place to make sure of it; and whether that
should be made more explicit in code than being picky about a type.

This commit has been cherry-picked from previous PR:

851c141

Author: veox

eth/vm/state.py

@@ -52,7 +52,7 @@ def __init__(
             self,
             db: AtomicDatabaseAPI,
             execution_context: ExecutionContextAPI,
-            state_root: bytes) -> None:
+            state_root: Hash32) -> None:
         self._db = db
         self.execution_context = execution_context
         self._account_db = self.get_account_db_class()(db, state_root)