input: Write a function to calculate the number of digits and letters in a string.
  assert dig_let("python")==(6,0)
  assert dig_let("program")==(7,0)
  assert dig_let("python3.0")==(6,2)
  
output: def dig_let(s):
      digits = sum(c.isdigit() for c in s)
      letters = sum(c.isalpha() for c in s)
      return letters, digits