input: Write a function that takes a string containing a numerical expression and returns the result.
  assert eval_expr("2+3")==5
  assert eval_expr("10-4")==6
  assert eval_expr("5*3")==15
  assert eval_expr("12/4")==3
  assert eval_expr("2**3")==8
output: def eval_expr(expression):
      return eval(expression)