macOS deploy: use the new plistlib API

See https://docs.python.org/3/library/plistlib.html.
The new API was added in 3.4 and old removed in 3.9.

contrib/macdeploy/macdeployqtplus

@@ -586,7 +586,8 @@ if len(config.fancy) == 1:
         sys.exit(1)
     
     try:
-        fancy = plistlib.readPlist(p)
+        with open(p, 'rb') as fp:
+            fancy = plistlib.load(fp, fmt=plistlib.FMT_XML)
     except:
         if verbose >= 1:
             sys.stderr.write("Error: Could not parse fancy disk image plist at \"{}\"\n".format(p))