Modernize legacy Python 2 syntax with pyupgrade.

Generated by `pyupgrade --py3-plus --keep-percent-format` followed by
manual indentation fixes.

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

zulip/integrations/svn/post-commit

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# -*- coding: utf-8 -*-
 #
 # Zulip notification post-commit hook.
 #
@@ -39,7 +38,7 @@ path, rev = sys.argv[1:]  # type: Tuple[Text, Text]
 path = "file://" + path
 
 entry = svn.log(path, revision_end=pysvn.Revision(pysvn.opt_revision_kind.number, rev))[0]  # type: Dict[Text, Any]
-message = "**{0}** committed revision r{1} to `{2}`.\n\n> {3}".format(
+message = "**{}** committed revision r{} to `{}`.\n\n> {}".format(
     entry['author'],
     rev,
     path.split('/')[-1],