Use a Protocol to ensure instance being serialized is a dataclass

[davetapley]

I just stumbled upon python/mypy#6568 (comment) and it made me think:
#312 is amazing, but it still (I presume) requires that the class being (de)serialized is a @dataclass?

If so then perhaps we could do:

class DataClassProtocol(Protocol):
    __dataclass_fields__: ClassVar[dict]

And then change o: Any here to o: DataClassProtocol:

pyserde/serde/se.py

Lines 433 to 434 in ad8c148

	def to_dict(
	o: Any,

Thoughts?