Fix open mode deprecated in Python 3.11

[Scotchester]

Python 3.11 deprecated the U flag when setting the mode in several file handling methods:

open(), io.open(), codecs.open() and fileinput.FileInput no longer accept 'U' (“universal newline”) in the file mode. In Python 3, “universal newline” mode is used by default whenever a file is opened in text mode, and the 'U' flag has been deprecated since Python 3.3. The newline parameter to these functions controls how universal newlines work. (Contributed by Victor Stinner in bpo-37330.)

This PR removes that flag where it was used in the divio app develop command.