Add __enter__ and __exit__ methods on Optimize class

This enables the use of the with statement for the Optimize class to
concisely call push() and pop(). This works similarly to the Solver
class.

src/api/python/z3/z3.py

@@ -7991,6 +7991,13 @@ class Optimize(Z3PPObject):
         if self._on_models_id is not None:
             del _on_models[self._on_models_id]
 
+    def __enter__(self):
+        self.push()
+        return self
+
+    def __exit__(self, *exc_info):
+        self.pop()
+
     def set(self, *args, **keys):
         """Set a configuration option.
         The method `help()` return a string containing all available options.