Here's an alternate patch that uses a bit of dependency injection to
avoid the need for monkeypatches in setup/teardown. This means some
trivial changes to Distribution.__init__().  I slightly prefer this
approach, but some might argue it's an example of "test logic in
production".

I also added a line about the new option in Doc/install/index.rst.

Since I don't have checkin privileges, I will stop here. Can somebody
upstream (Martin?) please take one of these patches and apply it? Or
suggest further changes to either of these patches?  Or ... ?
Thanks.