style: use import io everywhere

Align with Google style guides and `import io` as a module
rather than import symbols out of it.

Change-Id: Ic3e78aa81ffc24687f61c7ce744e596fbdb13415
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/654806
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

.flake8

@@ -52,7 +52,6 @@ per-file-ignores =
     tests/test_command.py: IMR241,
     tests/test_editor.py: IMR241,
     tests/test_error.py: IMR241,
-    tests/test_hooks.py: IMR241,
     tests/test_main.py: IMR241,
     tests/test_repo_logging.py: IMR241,
     tests/test_subcmds.py: IMR241,

tests/test_hooks.py

@@ -14,7 +14,7 @@
 
 """Unittests for the hooks.py module."""
 
-from io import StringIO
+import io
 from pathlib import Path
 import sys
 
@@ -82,7 +82,7 @@ def test_post_sync_argument_validation() -> None:
     )
 
     old_stderr = sys.stderr
-    sys.stderr = StringIO()
+    sys.stderr = io.StringIO()
 
     try:
         # Call with missing arg `sync_duration_seconds`