`is_subtype` fails with `List[TypeVar]`

[betodealmeida]

from typing import List, TypeVar
from pytypes import is_subtype

T = TypeVar("T")
is_subtype(List[int], List[T])

Fails with (Python 3.9):

Traceback (most recent call last):
  File "/Users/beto/Projects/datajunction/bug.py", line 5, in <module>
    is_subtype(List[int], List[T])
  File "/Users/beto/.pyenv/versions/dj/lib/python3.9/site-packages/pytypes/type_util.py", line 2738, in _issubclass
    res = _issubclass_2(
  File "/Users/beto/.pyenv/versions/dj/lib/python3.9/site-packages/pytypes/type_util.py", line 2814, in _issubclass_2
    return _issubclass_Generic(
  File "/Users/beto/.pyenv/versions/dj/lib/python3.9/site-packages/pytypes/type_util.py", line 1935, in _issubclass_Generic
    assert len(superclass.__args__) == len(prms)
AssertionError

And in 3.8:

Traceback (most recent call last):
  File "Projects/datajunction/src/datajunction/functions.py", line 89, in <module>
    print(infer_type("first", {"column": List[float]}) == float)
  File "Projects/datajunction/src/datajunction/functions.py", line 63, in infer_type
    if argument not in anno or not is_subtype(type_, anno[argument], type_):
  File "/Users/beto/.pyenv/versions/superset/lib/python3.8/site-packages/pytypes/type_util.py", line 2019, in _issubclass
    res = _issubclass_2(subclass, superclass, bound_Generic, bound_typevars,
  File "/Users/beto/.pyenv/versions/superset/lib/python3.8/site-packages/pytypes/type_util.py", line 2051, in _issubclass_2
    return _issubclass_Generic(subclass, superclass, bound_Generic, bound_typevars,
  File "/Users/beto/.pyenv/versions/superset/lib/python3.8/site-packages/pytypes/type_util.py", line 1582, in _issubclass_Generic
    if not _issubclass(p_self, p_cls, bound_Generic, bound_typevars,
  File "/Users/beto/.pyenv/versions/superset/lib/python3.8/site-packages/pytypes/type_util.py", line 2015, in _issubclass
    if not subclass.__bound__ is None:
AttributeError: 'NoneType' object has no attribute '__bound__'