[Python-ideas] Add from __experimental__ import bla [was: Should we move to replace re with regex?]

[Dj Gilcrease]

In the thread about replacing re with regex someone mentioned adding
to __future__ which isnt a great idea as future APIs are already
solidified, they just live there to give developer time to adapt their
code. The idea of a __experimental__ area is good for any pep's or
stliib additions that are somewhat controversial (API isnt agreed on,
code may take a while to integrate properly, developer wants some time
to hash out any edge case bugs or API clarifications that may come up
in large scale testing, etc).

__experimental__ should emit a warning on import that says anything in
here may change or be removed at any time and should not be used in
stable code.

__experimental__ features should behave the same as __future__ in that
they can add new keywords or semantics to the existing language

__experimental__ features can move directly to the stlib or builtins
if they do not add new keywords and/or are backwards compatible with
the feature they are replacing. Otherwise they move into __future__
for how ever many releases are deemed reasonable time for developers
to adapt their code.