Meta: Add .git-blame-ignore-revs

[AA-Turner]

This adds a .git-blame-ignore-revs file to ignore (almost) every 'administrative' change in the PEPs repo, leaving users of git blame seeing the main meat of the commit history.

The types of ignores are:

• reStructuredText conversion (but only pure conversion, for example dde379d, 631ff7f, e2a9d17, 2679eb0, etc aren't included)
• Header normalisation or validation
• Syntax fixes
• typos/grammar
• Whitespace changes
• Mass changes, e.g. using :pep: or adding Topic headers

A

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📚 Documentation preview 📚: https://pep-previews--3431.org.readthedocs.build/

[JelleZijlstra]

I think this is too much. We should ignore only those commits that changed whole files but weren't meaningful in terms of content, like the reST conversion.

[JelleZijlstra]

This just adds these Topic lines, I don't see why this needs to be ignored.