python: Sort imports with isort.

Fixes #2665.

Regenerated by tabbott with `lint --fix` after a rebase and change in
parameters.

Note from tabbott: In a few cases, this converts technical debt in the
form of unsorted imports into different technical debt in the form of
our largest files having very long, ugly import sequences at the
start.  I expect this change will increase pressure for us to split
those files, which isn't a bad thing.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/db.py

@@ -1,9 +1,8 @@
 import time
-from psycopg2.extensions import cursor, connection
-from psycopg2.sql import Composable
+from typing import Any, Callable, Dict, Iterable, List, Mapping, Optional, Sequence, TypeVar, Union
 
-from typing import Callable, Optional, Iterable, Any, Dict, List, Union, TypeVar, \
-    Mapping, Sequence
+from psycopg2.extensions import connection, cursor
+from psycopg2.sql import Composable
 
 CursorObj = TypeVar('CursorObj', bound=cursor)
 Query = Union[str, Composable]