[issue38899] Document that virtual environment activation for fish should use `source`

[Brett Cannon]

Change by Brett Cannon <brett at python.org>:


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keywords: +patch
pull_requests: +16844
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17359

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38899>
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