settings_manager: Use raw string for regex

Avoids a warning about an invalid escape sequence:

    ./settings_manager.py:221:36: W605 invalid escape sequence '\-'
                regex = '([a-fA-F0-9]{2}[:|\-]?){6}'

Fixes openbmc/openbmc#3140

Change-Id: I3be3d27bf9505b104db53a3a93e26fdc2e79e4db
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/settings_manager.py b/settings_manager.py
index d778978..1743f10 100644
--- a/settings_manager.py
+++ b/settings_manager.py
@@ -218,7 +218,7 @@
                 IP(value)
 
             elif key.lower() == 'mac':
-                regex = '([a-fA-F0-9]{2}[:|\-]?){6}'
+                regex = r'([a-fA-F0-9]{2}[:|\-]?){6}'
                 self.validate_regex(regex, value)
 
             elif key.lower() == 'prefix':