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

[kachida]

@jakkdl , Raised an PR# for this change, could you please review it

#3295

[A5rocks]

FYI you don't have to ping them, they should automatically get notifications when a PR is made. I do, at least!