py3: test to check which commands run

This test helps us to keep track on the commands which runs to Python 3.
The full traceback is hidden. Thanks to Augie and Martijn to wrap it up
in four lines.

tests/test-check-py3-commands.t

@@ -0,0 +1,14 b''
+#require py3exe
+
+This test helps in keeping a track on which commands we can run on
+Python 3 and see what kind of errors are coming up.
+The full traceback is hidden to have a stable output.
+
+  $ for cmd in version debuginstall ; do
+  >   echo $cmd
+  >   $PYTHON3 `which hg` $cmd 2>&1 2>&1 | tail -1
+  > done
+  version
+  TypeError: str expected, not bytes
+  debuginstall
+  TypeError: str expected, not bytes