wafsamba: Do not use 'rU' as the 'U' is deprecated in Python 3.9

wafsamba: Do not use 'rU' as the 'U' is deprecated in Python 3.9

See https://docs.python.org/3.9/whatsnew/3.9.html#changes-in-the-python-api

"open(), io.open(), codecs.open() and fileinput.FileInput no longer accept 'U'
(“universal newline”) in the file mode. This flag was deprecated since Python
3.3. In Python 3, the “universal newline” is used by default when a file is
open in text mode. The newline parameter of open() controls how universal
newlines works."

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14266

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Feb  6 07:30:13 UTC 2020 on sn-devel-184