Speed up mypy regr test in CI (#15735)

Author: JelleZijlstra

.github/workflows/tests.yml

@@ -71,9 +71,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
-          python-version: "3.15"
-          allow-prereleases: true
-          check-latest: true
+          python-version: "3.14"
       - uses: astral-sh/setup-uv@v7
         with:
           version-file: "requirements-tests.txt"

tests/regr_test.py

@@ -358,7 +358,8 @@ def cleanup_threads(
         ]
 
         with cleanup_threads(event, printer_thread, executor):
-            concurrent.futures.wait(testcase_futures)
+            for future in concurrent.futures.as_completed(testcase_futures):
+                future.result()
 
         mypy_futures = [executor.submit(task) for task in to_do]