Fix compilation issue in fedora 37/rawhide.

Compilation is failing with the error:

File "Documentation/conf.py", line 61, in <module>
    with open(filename, 'rU') as f:
         ^^^^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'rU'

The python 3 documentation [1] says:

There is an additional mode character permitted, 'U', which no
longer has any effect, and is considered deprecated. It
previously enabled universal newlines in text mode, which
became the default behaviour in Python 3.0. Refer to the
documentation of the newline parameter for further details.

[1] - https://docs.python.org/3.9/library/functions.html#open

This patch fixes this issue.

Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Numan Siddique <numans@ovn.org>
(cherry picked from commit 500982b)

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, bugs@openvswitch.org)":