PEP 704: Withdraw and add a withdrawal notice

[pradyunsg]

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--3081.org.readthedocs.build/

[CAM-Gerlach]

LGTM, thanks @pradyunsg 👍

(Personal comment: While a PEP being withdrawn is always at least a little disappointing, this PEP still was quite fruitful in that it prompted a lot of great discussion and ideas that has helped improve the understanding of the current situation here and guide efforts to improve it (for example, implementation of EXTERNALLY-MANAGED in Conda, and a good deal of further thought on how to improve pip- and other-tool interoperability in that regard. Furthermore, it will hopefully lead to more standardization in venv naming, creation and usage in the future, which will help address much of the remaining motivation. Thanks!).