util: flush stderr explicitly after using warnings.warn()

Due to some unknown reasons, when using chg with python3, the warnings.warn()
output is not flushed.

Fixes test-devel-warnings.t on py3 with chg.

mercurial/util.py

@@ -204,6 +204,8 b' def nouideprecwarn(msg, version, stackle'
             b" update your code.)"
         ) % version
         warnings.warn(pycompat.sysstr(msg), DeprecationWarning, stacklevel + 1)
+        # on python 3 with chg, we will need to explicitly flush the output
+        sys.stderr.flush()
 
 
 DIGESTS = {