[docs] [issue35106] Add documentation for `type.__subclasses__` to docs.python.org

[Joy Diamond]

New submission from Joy Diamond <python.gem at gmail.com>:

Add documentation for `type.__subclasses__` to docs.python.org

Python ideas discussion:

https://mail.python.org/pipermail/python-ideas/2018-October/054361.html

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assignee: docs at python
components: Documentation
messages: 328848
nosy: docs at python, joydiamond
priority: normal
severity: normal
status: open
title: Add documentation for `type.__subclasses__` to docs.python.org
versions: Python 2.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35106>
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