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/realm_icon.py

@@ -4,6 +4,7 @@ from zerver.lib.avatar_hash import gravatar_hash
 from zerver.lib.upload import upload_backend
 from zerver.models import Realm
 
+
 def realm_icon_url(realm: Realm) -> str:
     return get_realm_icon_url(realm)