Validate the bpo number

[Mariatta]

The bpo number provided in python/cpython#1798 is not valid (46895)
https://bugs.python.org/issue46895 is a 404

Maybe bedevere can also validate whether the issue number is correct :)

[brettcannon]

Yeah, we could do a simple GET request and see if the response is a 404 or not to flag the issue number as formatted properly but not an actual issue number.

[serhiy-storchaka]

Doing a HEAD request may be enough.

[lysnikolaou]

This should be closed since #159 is now merged.