setup.py: remove the 'U' open mode removed in Python 3.11

[jolivain]

The 'U' open mode was removed in Python 3.11. See: https://docs.python.org/3.11/library/functions.html#open

Installing with Python 3.11 fails with:

/path/to/python-magic-wormhole-mailbox-server-0.4.1/setup.py", line 20, in <module>
    long_description=open('README.md', 'rU').read(),
                     ^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'rU'

This patch removes the 'U' open mode.

Signed-off-by: Julien Olivain ju.o@free.fr

[codecov[bot]]

Codecov Report

Base: 97.10% // Head: 97.10% // No change to project coverage :thumbsup:

Coverage data is based on head (df6126e) compared to base (ac91c3a). Patch has no changes to coverable lines.

Additional details and impacted files

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[meejah]

thanks!