Replace deprecated readfp method with read_file

The readfp method has been deprecated since version 3.2 [1].

[1] https://docs.python.org/3/library/configparser.html?highlight=deprecated#configparser.ConfigParser.readfp

Change-Id: Ib4d13e850e47c80ca6ec601b9e195337d9a403e0

doc/ext/support_matrix.py

@@ -101,7 +101,7 @@ class SupportMatrixDirective(rst.Directive):
                                  "support-matrix.ini")
         rel_fpath, fpath = env.relfn2path(fname)
         with open(fpath) as fp:
-            cfg.readfp(fp)
+            cfg.read_file(fp)
 
         # This ensures that the docs are rebuilt whenever the
         # .ini file changes