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/reddit.py

@@ -1,3 +1,4 @@
+import dataclasses
 import datetime
 import logging
 import re
@@ -13,7 +14,8 @@ logger = logging.getLogger(__name__)
 
 # Most of these fields should never be None, but due to broken data, they sometimes are anyway...
 
-class Submission(typing.NamedTuple, snscrape.base.Item):
+@dataclasses.dataclass
+class Submission(snscrape.base.Item):
 	author: typing.Optional[str] # E.g. submission hf7k6
 	created: datetime.datetime
 	id: str
@@ -27,7 +29,8 @@ class Submission(typing.NamedTuple, snscrape.base.Item):
 		return self.url
 
 
-class Comment(typing.NamedTuple, snscrape.base.Item):
+@dataclasses.dataclass
+class Comment(snscrape.base.Item):
 	author: typing.Optional[str]
 	body: str
 	created: datetime.datetime