Incompatible `AwareDateTime` with pytz

Hello!

As I was trying to introduce strict typing to my project I ran into an issue when integrating pytz with marshmallow. It seems that marshmallow might be requesting too strict of types. The request for only datetime timezones prevents me from using pytz with the project (even though they are compatible). The mypy maintainer suggested that marshmallow needs to integrate protocol types to be able to solve this problem, though I’ll admit I’m not too sure what that means in this case. For a rundown of the issue please see:

https://github.com/python/mypy/issues/8632

[adithyabsk]

I would mark this issue as a bug @deckar01 I’ll try to implement @lafrech patch and submit a PR when I get a chance

[lafrech]

My point was that perhaps we should specify datetime.tzinfo as argument type rather than datetime.timezone.

    def __init__(
        self, format: str = None, *, default_timezone: dt.tzinfo = None, **kwargs
    ):

What we actually expect is a tzinfo, not specifically a dt.timezone.

dt.timezone is a convenience class provided by Python core to implement basic timezones. pytz timezones also inherit from tzinfo, which is the right thing to do, not dt.timezone.

The change above should fix the issue IIUC.