Bug 1769631 - Remove 'U' from 'mode' parameters for various 'open' calls to ensure...

Bug 1769631 - Remove 'U' from 'mode' parameters for various 'open' calls to ensure Python3.11 compatibility r=firefox-build-system-reviewers,glandium

The 'U' flag represents "universal newline". It has been deprecated
since Python3.3. Since then "universal newline" is the default when a
file is opened in text mode (not bytes). In Python3.11 using the 'U'
flag throws errors. There should be no harm in removing 'U' from 'open'
everywhere it is used, and doing allows the use of Python3.11.

For more reading see: https://docs.python.org/3.11/whatsnew/3.11.html#changes-in-the-python-api

Differential Revision: https://phabricator.services.mozilla.com/D147721

Gbp-Pq: Topic fixes
Gbp-Pq: Name Bug-1769631-Remove-U-from-mode-parameters-for-variou.patch