Revert relative imports for non-module sandbox settings

c06e1df wrongly changed the imports to relative imports, as settings
isn't a module (but probably should be). Reverting those changes for
now.

sites/sandbox/settings_mysql.py

-from .settings import *
+from settings import *
 
 DATABASES = {
     'default': {

sites/sandbox/settings_postgres.py

-from .settings import *
+from settings import *
 
 DATABASES = {
     'default': {

sites/sandbox/settings_sphinx.py

-from .settings import *
+from settings import *
 
 # Remove debug toolbar
 try: