input: Write a function to create a Python function that takes a string containing a numerical expression and returns the result.
  assert eval_expr("2+3")==5
  assert eval_expr("5*7")==35
  assert eval_expr("10/2")==5.0
  assert eval_expr("12-4")==8
  assert eval_expr("3**2")==9
output: def eval_expr(expression):
      return eval(expression)