[docs] [issue27285] Document the deprecation of pyvenv in favor of `python3 -m venv`

[Roundup Robot]

Roundup Robot added the comment:

New changeset 2a34cef7681b by Ned Deily in branch 'default':
Issue #27285: Cleanup "suspicious" warnings.
https://hg.python.org/cpython/rev/2a34cef7681b

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27285>
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