isort: format codebase

Change-Id: I6f11d123b68fd077f558d3c21349c55c5f251019
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/383715
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

fetch.py

@@ -18,6 +18,7 @@ import subprocess
 import sys
 from urllib.parse import urlparse
 from urllib.request import urlopen
+
 from error import RepoExitError