mypy: Set local_partial_types = True.

The daemon implicitly sets this flag, so set it explicitly and fix all
of the type errors.

zerver/lib/sqlalchemy_utils.py

@@ -1,3 +1,5 @@
+from typing import Optional, Any
+
 from django.db import connection
 from zerver.lib.db import TimeTrackingConnection
 
@@ -21,7 +23,7 @@ class NonClosingPool(sqlalchemy.pool.NullPool):
                               logging_name=self._orig_logging_name,
                               _dispatch=self.dispatch)
 
-sqlalchemy_engine = None
+sqlalchemy_engine = None  # type: Optional[Any]
 def get_sqlalchemy_connection() -> sqlalchemy.engine.base.Connection:
     global sqlalchemy_engine
     if sqlalchemy_engine is None: