zerver/lib: Use python 3 syntax for typing.

Split by tabbott from a larger commit; this covers a batch of files
with no open PRs touching them.

zerver/lib/sqlalchemy_utils.py

@@ -6,16 +6,13 @@ import sqlalchemy
 # This is a Pool that doesn't close connections.  Therefore it can be used with
 # existing Django database connections.
 class NonClosingPool(sqlalchemy.pool.NullPool):
-    def status(self):
-        # type: () -> str
+    def status(self) -> str:
         return "NonClosingPool"
 
-    def _do_return_conn(self, conn):
-        # type: (sqlalchemy.engine.base.Connection) -> None
+    def _do_return_conn(self, conn: sqlalchemy.engine.base.Connection) -> None:
         pass
 
-    def recreate(self):
-        # type: () -> NonClosingPool
+    def recreate(self) -> 'NonClosingPool':
         return self.__class__(creator=self._creator,
                               recycle=self._recycle,
                               use_threadlocal=self._use_threadlocal,
@@ -25,12 +22,10 @@ class NonClosingPool(sqlalchemy.pool.NullPool):
                               _dispatch=self.dispatch)
 
 sqlalchemy_engine = None
-def get_sqlalchemy_connection():
-    # type: () -> sqlalchemy.engine.base.Connection
+def get_sqlalchemy_connection() -> sqlalchemy.engine.base.Connection:
     global sqlalchemy_engine
     if sqlalchemy_engine is None:
-        def get_dj_conn():
-            # type: () -> TimeTrackingConnection
+        def get_dj_conn() -> TimeTrackingConnection:
             connection.ensure_connection()
             return connection.connection
         sqlalchemy_engine = sqlalchemy.create_engine('postgresql://',