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>

tools/tests/test_html_branches.py

@@ -1,13 +1,12 @@
-import unittest
 import os
+import unittest
 
 import tools.lib.template_parser
-
 from tools.lib.html_branches import (
+    build_id_dict,
     get_tag_info,
     html_branches,
     html_tag_tree,
-    build_id_dict,
     split_for_id_and_class,
 )