Please consider dropping obsolete "U" mode passed to open()

[pgrt]

Hello,

There are four places in the source code of sepp 4.5.1 (and also at the current tip of the master branch) where open is used with the "rU" speficier.

The "U" mode has been deprecated in Python since Python3 and has had no effect. In Python 3.11, it is now disallowed. This mode flag causes runtime failures.

Link to Debian bug tracker about this for information: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029580

Best,

-- Pierre