Fix: spinner using invalid escape sequence (#298)

Before this running this code would result in:
```
maas/client/utils/__init__.py:341: SyntaxWarning: invalid escape sequence '\|'
```

Co-authored-by: Jacopo Rota <jacopo.rota@canonical.com>

maas/client/utils/__init__.py

@@ -338,7 +338,7 @@ class Spinner:
     Use as a context manager.
     """
 
-    def __init__(self, frames="/-\|", stream=sys.stdout):
+    def __init__(self, frames=r"/-\|", stream=sys.stdout):
         super(Spinner, self).__init__()
         self.frames = frames
         self.stream = stream