Drop attrs and use dataclass instead

[pepoluan]

Another thought occurred to me: As we've dropped Python 3.6 support, we can also use dataclass rather than attrs.

This will reduce aiosmtpd's dependency for sure.

[pepoluan]

I have found that attrs's kw_only field specifier is widely used, and dataclass didn't begin to have the same feature until 3.10

So... keep this dep for now?

[waynew]

That's probably fair enough. I don't actually even know attrs or dataclass that well, unfortunately.