[issue39674] Keep deprecated features in Python 3.9 to ease migration from Python 2.7, but remove in Python 3.10

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

Another candidate is to revert the ignored "U" mode in open(): commit e471e72977c83664f13d041c78549140c86c92de of bpo-37330.

Removing "U" mode of open() broke 11 packages in Fedora:

* aubio
* openvswitch
* python-SALib
* python-altgraph
* python-apsw
* python-magic-wormhole-mailbox-server
* python-munch
* python-parameterized
* python-pylibmc
* python-sphinx-testing
* veusz

Keeping "U" mode in Python 3.9 is also a formal request from Andrew Bartlett of the Samba project: https://bugs.python.org/issue37330#msg362362

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