[issue36404] Document PendingDeprecationWarning as deprecated

[Inada Naoki]

New submission from Inada Naoki <songofacandy at gmail.com>:


New changeset 176d26364bb67801fa522f52f20cbe44420d6942 by Inada Naoki in branch 'master':
bpo-36404: recommend DeprecationWarning over PendingDeprecationWarning (GH-12505)
https://github.com/python/cpython/commit/176d26364bb67801fa522f52f20cbe44420d6942


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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36404>
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