Fix the leaking file descriptor when returning a z3 smt check result (#1574)

mythril/laser/smt/solver/solver.py

@@ -54,7 +54,8 @@ class BaseSolver(Generic[T]):
         :return: The evaluated result which is either of sat, unsat or unknown
         """
         old_stdout = sys.stdout
-        sys.stdout = open(os.devnull, "w")
+        with open(os.devnull, "w") as dev_null_fd:
+            sys.stdout = dev_null_fd
             try:
                 evaluate = self.raw.check(args)
             except z3.z3types.Z3Exception as e: