[stdlib-sig] standardizing the deprecation policy (and how noisy they are)

[Brett Cannon]

During the moratorium PEP discussions Guido said he wanted to quiet
down deprecation warnings. I see there being two options on this.

One is to keep things as is, but to require two releases with
PendingDeprecationWarning so there are three years of
silent-by-default warnings to update your code. But that last release
before removal came would still be noisy.

The other option is to simply have all warnings filtered out by
default. We could alter -W so that when it is used w/o an argument it
turns to what is currently the default behaviour (or even turn all
warnings which is more than what happens now). This will require that
people proactively check for warnings when updating for compatibility,
else they will eventually use a Python release where there code will
simply break because something changed. This route means we do not
have to specify any deprecation policy right now (that would be a
separate discussion).

Channeling Guido he is after the latter, but a general discussion
would still be good since he didn't explicitly say what he was after
other than to quiet down warnings.

-Brett