check-py3-compat: manually format and print warnings

The warnings mechanism may print to stderr on Python 3. Independent buffering
of stdout and stderr can lead to warnings output not being printed properly.

This commit traps warnings when executing individual files and prints
warnings to stdout so output is deterministic.

Differential Revision: https://phab.mercurial-scm.org/D5845

contrib/check-py3-compat.py

@@ -14,6 +14,7 b' import importlib'
 import os
 import sys
 import traceback
+import warnings
 
 def check_compat_py2(f):
     """Check Python 3 compatibility for a file with Python 2"""
@@ -91,6 +92,11 b" if __name__ == '__main__':"
         fn = check_compat_py3
 
     for f in sys.argv[1:]:
-        fn(f)
+        with warnings.catch_warnings(record=True) as warns:
+            fn(f)
+
+        for w in warns:
+            print(warnings.formatwarning(w.message, w.category,
+                                         w.filename, w.lineno).rstrip())
 
     sys.exit(0)