[docs] [issue35581] Document @typing.type_check_only

[Sebastian Rittau]

New submission from Sebastian Rittau <srittau at rittau.biz>:

Document @typing.type_check_only per https://github.com/python/typing/issues/597.

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assignee: docs at python
components: Documentation
messages: 332508
nosy: docs at python, srittau
priority: normal
severity: normal
status: open
title: Document @typing.type_check_only
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35581>
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