filter-repo: fix import sort order

During the python3 transition, StringIO was renamed to io -- but the
import wasn't moved to preserve appropriate sorting.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -34,10 +34,10 @@ import argparse
 import collections
 import fnmatch
 import gettext
+import io
 import os
 import re
 import shutil
-import io
 import subprocess
 import sys
 import time