Too-narrow validator for :pep: role

[Zac Hatfield-Dodds]

    :pep:`Python 3.4 end-of-life <429>`

is a valid and useful bit of Sphinx markup, but triggers the D000 error.

I think the general solution is to improve parsing of the roles to support the link text / link target distinction, as this may be disguising latent issues elsewhere in the code.