bpo-27103: regrtest disables -W if -R is used

[vstinner]

Workaround for a regrtest bug.

[mention-bot]

@Haypo, thanks for your PR! By analyzing the history of the files in this pull request, we identified @brettcannon, @zooba and @abalkin to be potential reviewers.

[vstinner]

This change is not a proper fix, just a workaround to be able to move forward on the new "Refleaks" buildbot that @zware just created:
http://buildbot.python.org/all/builders/x86%20Gentoo%20Refleaks%203.6/

[zware]

I think I'd rather change the builder config, possibly by adding a new make target for it (make refleaktest?).

[vstinner]

I was bitten multiple timed by this bug. The rationale is non obvious. I would like to fix it.

[zware]

Ok, fair enough.

[serhiy-storchaka]

LGTM. Could you add a test for this? It should be easy in ParseArgsTestCase.

I don't know whether it is worth to make this an error rather than a warning.

[vstinner]

Could you add a test for this? It should be easy in ParseArgsTestCase Since it's a temporary workaround, I don't want to write a test for it. I'm quite sure that it's possible to really fix the bug by reorganizing the code.