Replace named tuples with dataclasses and move JSON conversion logic to the base classes

Named tuples were never really adequate for this since the order aspect of them doesn't make sense.
Further, named tuples don't support multiple inheritance. This meant that the objects returned by get_items() were not actually Items, for example. Since Python 3.9, such named tuples cannot be created anymore.

Fixes #111

snscrape/modules/vkontakte.py

@@ -1,5 +1,6 @@
 import bs4
 import collections
+import dataclasses
 import datetime
 import itertools
 import logging
@@ -11,7 +12,8 @@ import urllib.parse
 logger = logging.getLogger(__name__)
 
 
-class VKontaktePost(typing.NamedTuple, snscrape.base.Item):
+@dataclasses.dataclass
+class VKontaktePost(snscrape.base.Item):
 	url: str
 	date: datetime.datetime
 	content: str
@@ -20,7 +22,8 @@ class VKontaktePost(typing.NamedTuple, snscrape.base.Item):
 		return self.url
 
 
-class User(typing.NamedTuple, snscrape.base.Entity):
+@dataclasses.dataclass
+class User(snscrape.base.Entity):
 	username: str
 	name: str
 	verified: bool