presubmit: Update context references

These have been moved in upstream Pigweed: pwrev/151610.

Change-Id: Ie96a152f14253bebfa60590bb3a1d03972564962
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/151671
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/tools/pigweed_experimental_tools/presubmit_checks.py b/tools/pigweed_experimental_tools/presubmit_checks.py
index f888d0b..d5d5a75 100755
--- a/tools/pigweed_experimental_tools/presubmit_checks.py
+++ b/tools/pigweed_experimental_tools/presubmit_checks.py
@@ -40,8 +40,8 @@
     install_hook,
     keep_sorted,
     python_checks,
-    PresubmitContext,
 )
+from pw_presubmit.presubmit_context import PresubmitContext, PresubmitFailure
 
 _LOG = logging.getLogger(__name__)
 
@@ -154,7 +154,7 @@
     if changes:
         _LOG.warning(
             'Commit or stash pending changes before running the presubmit.')
-        raise pw_presubmit.PresubmitFailure
+        raise PresubmitFailure
 
 
 # Avoid running some checks on certain paths.