PEP 642: Fix footnotes

[hugovk]

Fix these warnings:

pep-0642.rst:1835: WARNING: Footnote [1] is not referenced.
pep-0642.rst:1838: WARNING: Footnote [2] is not referenced.
pep-0642.rst:1841: WARNING: Footnote [3] is not referenced.
pep-0642.rst:1844: WARNING: Footnote [4] is not referenced.
pep-0642.rst:1847: WARNING: Footnote [5] is not referenced.
pep-0642.rst:1850: WARNING: Footnote [6] is not referenced.
pep-0642.rst:1853: WARNING: Footnote [7] is not referenced.
pep-0642.rst:1856: WARNING: Footnote [8] is not referenced.
pep-0642.rst:1859: WARNING: Footnote [9] is not referenced.

Like #2714, the footnotes in the text should be of the form [1]_ not [1_].

• https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#footnotes

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And fix this:

pep-0400.txt:213: WARNING: duplicate label appendix a, other instance in pep-0642.rst

Sphinx treats labels as "global" in the whole docs scope, not at the level of individual documents. So let's use a unambiguous label prefixed with the PEP number, similar to PEP 639 / https://raw.githubusercontent.com/python/peps/main/pep-0639.rst

And there's still an #appendix-a anchor:

• main: https://peps.python.org/pep-0642/#appendix-a
• PR: https://pep-previews--2718.org.readthedocs.build/pep-0642/#appendix-a

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Also remove redundant emacs metadata.

Preview

https://pep-previews--2718.org.readthedocs.build/pep-0642/