Add a script to linkify PEPs

[encukou]

Here is a rather crude script that adds Markdown links and titles to “PEP nnn”. Maybe it'll be useful.
I meant to polish this before sharing it, but didn't get around to it, so it has sharp edges. If you use it, only feed it backed-up files, and check its work.

[StanFromIreland]

Note that if one writes PEP-XXX in a GitHub issue, the link is automatically created, e.g.: PEP-545.