Cannot type check with `from __future__ import annotations`

`from __future__ import annotations` turns type annotations into strings that can later be evaluated at runtime via `typing.get_type_hints` see [PEP 563](https://peps.python.org/pep-0563/). This is problematic because TestSlide uses the following pattern to find the expected type:
```
try:
argspec = inspect.getfullargspec(template)
except TypeError:
return
expected_type = argspec.annotations.get("return")
```
In this scenario, expected_type is a `string` when the expected type is `Type` (see `_validate_argument_type()` which only works if expected_type is a `Type`).

It seems like a potential solution is:
```
try:
hints = get_type_hints(template)
expected_type = hints.get("return")
except TypeError:
return
```
Will continue testing (I am on python 3.8.6).