input: Write a function to check if a string can be converted to a number.
  assert is_number("123.45") == True
  assert is_number("abc") == False
  assert is_number("0.99") == True
  assert is_number("123abc") == False
output: def is_number(s):
      try:
          float(s)
          return True
      except ValueError:
          return False