SEDbgMuxApp: use cmd2.Fg instead of deprecated cmd2.fg

In v2.3.0 both cmd2.{fg,bg} have been deprecated in favour of cmd2.{Fg,Bg}.
In v2.4.0 both cmd2.{fg,bg} have been removed.

See https://github.com/python-cmd2/cmd2/blob/master/CHANGELOG.md

sedbgmux.py

@@ -39,7 +39,7 @@ class SEDbgMuxApp(cmd2.Cmd):
     def __init__(self, argv):
         super().__init__(allow_cli_args=False)
 
-        self.intro = cmd2.style('Welcome to %s!' % self.DESC, fg=cmd2.fg.red)
+        self.intro = cmd2.style('Welcome to %s!' % self.DESC, fg=cmd2.Fg.RED)
         self.prompt = 'DebugMux (\'%s\')> ' % argv.serial_port
         self.default_category = 'Built-in commands'
         self.argv = argv