FYI, instead of trying to do exhaustive type checking in _check_type(),
you can just rely on duck typing and catch the TypeError:

>>> str(b"a", "utf-8")
'a'
>>> str(bytearray(b"a"), "utf-8")
'a'
>>> str(memoryview(b"a"), "utf-8")
'a'
>>> str(1, "utf-8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: coercing to str: need string or buffer, int found