This commit resolves all outstanding duplicate label
and duplicate citation warnings.
I have not yet addressed unreferenced footnote warnings.
I took the general strategy of replacing a target above a section heading
with references directly to that section heading,
or where impractical, disambiguting the target with ``PEP NNN``.
I also converted all the citations in PEP 482 to links,
as they were causing duplicate citation warnings.
Adam Turner
Brett Cannon