Accept `None` as return type

[raabf]

You should allow to write something like that to indicate there is no return (=void)

@typechecked
def rettest(self) -> None:
    print("Hello World")

But Actually

Hello World
Traceback (most recent call last):
[...]
  File "[...]/annotation/typed.py", line 500, in wrapper
    return _check_return_type(signature, target(*args, **kwargs))
  File "[...]/annotation/typed.py", line 477, in _check_return_type
    raise TypeError('Incorrect return type')
TypeError: Incorrect return type

is thrown and you had to use

@typechecked
def rettest(self) -> type(None):
    print("Hello World")

but writing that is ugly and PEP484 defines that

When used in a type hint, the expression None is considered equivalent to type(None) .

so you should also allow None as type.

This is an alternative to #8 .