Yes, I wrote the previous message from a cellphone and I wasn't able to
check the doc. It is indeed already documented in datetime.__str__ --
sorry for the noise.
I also noticed that the microseconds are not the only thing that can
change in datetime.isoformat(), there are also the separator and the UTC
offset, so if someone is parsing the output of str(d)/d.isoformat(), he
should already include other checks anyway.

I agree that only the test should be fixed (and possibly others tests
should be added to check that str() outputs the right thing when there
is a UTC offset and/or a different separator).

I'll work on a patch for that.