input: Write a python function to get all possible substrings of a given string.
  assert get_all_substrings('Python') == ['P', 'Py', 'Pyt', 'Pyth', 'Pytho', 'Python', 'y', 'yt', 'yth', 'ytho', 'ython', 't', 'th', 'tho', 'thon', 'h', 'ho', 'hon', 'o', 'on', 'n']
  assert get_all_substrings('abc') == ['a', 'ab', 'abc', 'b', 'bc', 'c']
  assert get_all_substrings('a') == ['a']
output: def get_all_substrings(s):
      substrings = []
      for i in range(len(s)):
          for j in range(i + 1, len(s) + 1):
              substrings.append(s[i:j])
      return substrings