[issue39712] Doc for `-X dev` option should mention PYTHONDEVMODE

[Erlend Egeberg Aasland]

Change by Erlend Egeberg Aasland <erlend.aasland at innova.no>:


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nosy: +erlendaasland
nosy_count: 9.0 -> 10.0
pull_requests: +18045
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/18685

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39712>
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