use :pep: in rst

[jakkdl]

          FYI, Sphinx has a `:pep:` role that one can use for PEP links in RST.

Originally posted by @webknjaz in #3151 (comment)

I'm finding two more instances in current documentation:

$ git grep PEP
docs/source/history.rst:  (the type of object sent over the channel or produced by the listener) using PEP 484 syntax:
docs/source/reference-lowlevel.rst:going straight to `PEP 492