Add __enter__ and __exit__ methods to Solver class (#7025)

To enable the usage of the with statement for the Solver class
instances, this commit adds __enter__ and __exit__ methods.
The with statement should offer a more concise and safer
alternative to explicit usage of push() and pop() methods.

src/api/python/z3/z3.py

@@ -6955,6 +6955,13 @@ def __del__(self):
         if self.solver is not None and self.ctx.ref() is not None and Z3_solver_dec_ref is not None:
             Z3_solver_dec_ref(self.ctx.ref(), self.solver)
 
+    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.