input: Write a python function to delete keys from a dictionary whose value is equal to a given value.
  assert delete_keys({'a':1, 'b':2, 'c':3, 'd':2}, 2) == {'a': 1, 'c': 3}
  assert delete_keys({'x':5, 'y':6, 'z':5}, 5) == {'y': 6}
  assert delete_keys({'m':10, 'n':20, 'o':30, 'p':20}, 20) == {'m': 10, 'o': 30}
output: def delete_keys(d, value_to_delete):
      return {k: v for k, v in d.items() if v != value_to_delete}