Add support for MappingProxyType

[Fatal1ty]

This adds support for types.MappingProxyType:

from types import MappingProxyType

from mashumaro.codecs import BasicDecoder, BasicEncoder

decoder = BasicDecoder(MappingProxyType[int, float])
encoder = BasicEncoder(MappingProxyType[int, float])

assert repr(decoder.decode({"42": "42"})) == "mappingproxy({42: 42.0})"
assert encoder.encode(MappingProxyType({42: 42.0})) == {42: 42.0}

from dataclasses import dataclass
from types import MappingProxyType

from mashumaro import DataClassDictMixin

@dataclass
class MyClass(DataClassDictMixin):
    x: MappingProxyType[int, float]

obj = MyClass.from_dict({"x": {"42": "42"}})
assert repr(obj) == "MyClass(x=mappingproxy({42: 42.0}))"
assert repr(obj.to_dict()) == "{'x': {42: 42.0}}"