Fix ActivityIndicator context entering (#197)

This commit enables statements like...

    with ActivityIndicator(view) as i:
       ...

Author: deathaxe

sublime_lib/activity_indicator.py

@@ -89,8 +89,9 @@ def __init__(
     def __del__(self) -> None:
         self._target.clear()
 
-    def __enter__(self) -> None:
+    def __enter__(self) -> ActivityIndicator:
         self.start()
+        return self
 
     def __exit__(
         self,