Tweak PEP 387 to recommend deprecating for 5 years

[brettcannon]

https://discuss.python.org/t/updating-pep-387-to-prefer-5-year-deprecations-instead-of-2-years/65166

[Yhg1s]

PR?

[brettcannon]

PR?

python/peps#4156

[emilyemorehouse]

Closing this since the related PR has been approved and merged!