Python adopts a 12-month release cycle

On behalf of the steering council I am happy to announce that as
BDFL-Delegate I am accepting PEP 602 to move us to an annual release schedule
(gated on a planned update; see below).

The steering council thinks that having a consistent schedule every year when
we hit beta, RC, and final it will help the community:

* Know when to start testing the beta to provide feedback

* Known when the expect the RC so the community can prepare their projects
for the final release

* Know when the final release will occur to coordinate their own releases (if
necessary) when the final release of Python occurs

* Allow core developers to more easily plan their work to make sure work
lands in the release they are targeting

* Make sure that core developers and the community have a shorter amount of
time to wait for new features to be released

The acceptance is gated on Łukasz updating PEP 602 to reflect a planned shift
in scheduling (he's been busy with a release of Black):

* 3 months for betas instead of 2

* 2 months for RCs instead of 1

This was discussed on https://discuss.python.org in order to give the
community enough time to provide feedback in the betas while having enough
time to thoroughly test the RC and to prep for the final release so the delay
from Python's final release to any new project releases is minimal. It should
also fit into the release schedule of Linux distributions like Fedora better
than previously proposed so the distributions can test the RC when they start
preparing for their own October releases. If this turns out to be a mistake
after we try it out for Python 3.9 we can then discuss going back to longer
betas and shorter RCs for the release after that. This will not change when
feature development is cut off relative to PyCon US nor the core dev sprints
happening just before the final release or the alpha of the next version.

To help people who cannot upgrade on an annual cycle, do note that:

* PEP 602 now says that deprecations will last two releases which is two
years instead of the current 18 months

* Now that the stable ABI has been cleaned, extension modules should feel
more comfortable targeting the stable ABI which should make supporting newer
versions of Python much easier

As part of the shift to a 2 year deprecation time frame I will be restarting
discussions around PEP 387 as BDFL-Delegate so we can have a more clear
deprecation and backwards-compatibility policy as well for those that find an
annual cycle too fast which will be updated to reflect this two year time
frame (head's up, Benjamin 😉).

Thanks to Łukasz, Nick, and Steve for PEPs 602, 605, and 607 and everyone
else who provided feedback on those PEPs!
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