Issues type checking empty lists

[mitar]

>>> import typing
>>> from pytypes import type_util
>>> type_util._isinstance([], typing.Sequence)
True
>>> type_util._isinstance([], typing.Sequence[int])
False

False there is surprising. I understand why it happens: because internally isinstance is converted to issubclass and Sequence[Any] is not a subclass of Sequence[int].

[Stewori]

We use a special type type_util.Empty[T] to model empty sets, dicts, lists to avoid lacking covariance of mutable Any-parameters. So [] should become type_util.Empty[List] which should be subtype of Sequence[int]. Cannot tell right now why this fails, but have to investigate. (Currently busy with upcoming Python 3.7 support.)

[Stewori]

Fixed as of 8f0e738.
Todo: Add test.