[issue37312] Remove deprecated _dummy_thread and dummy_threading modules

[STINNER Victor]

New submission from STINNER Victor <vstinner at redhat.com>:

The _dummy_thread and dummy_threading modules are deprecated since Python 3.7 which now requires threading support. Attached PR removes these modules.

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components: Library (Lib)
messages: 345810
nosy: vstinner
priority: normal
severity: normal
status: open
title: Remove deprecated _dummy_thread and dummy_threading modules
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37312>
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