[issue40558] update CONTRIBUTING.rst docs

[hai shi]

New submission from hai shi <shihai1991 at 126.com>:

update the links of buildbot in CONTRIBUTING.rst.

I don't know why I can not visit `http://buildbot.python.org/3.8.stable/`

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assignee: docs at python
components: Documentation
messages: 368422
nosy: docs at python, shihai1991
priority: normal
severity: normal
status: open
title: update CONTRIBUTING.rst docs
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40558>
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