Documentation: Fix building with Python 3.9

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.

Acked-by: Flavio Leitner <[email protected]>
Reported-at: https://bugzilla.redhat.com/1791681
Reported-by: Miro Hrončok <[email protected]>
Signed-off-by: Timothy Redaelli <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

Documentation/conf.py

@@ -58,7 +58,7 @@
 # The full version, including alpha/beta/rc tags.
 release = None
 filename = "../configure.ac"
-with open(filename, 'rU') as f:
+with open(filename, 'r') as f:
     for line in f:
         if 'AC_INIT' in line:
             # Parse "AC_INIT(openvswitch, 2.7.90, [email protected])":