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/base.py

@@ -1,5 +1,8 @@
 import abc
+import dataclasses
+import datetime
 import functools
+import json
 import logging
 import requests
 import time
@@ -8,7 +11,28 @@ import time
 logger = logging.getLogger(__name__)
 
 
-class Item:
+def _json_serialise_datetime(obj):
+	'''A JSON serialiser that converts datetime.datetime and datetime.date objects to ISO-8601 strings.'''
+	if isinstance(obj, (datetime.datetime, datetime.date)):
+		return obj.isoformat()
+	raise TypeError(f'Object of type {type(obj)} is not JSON serializable')
+
+
+@dataclasses.dataclass
+class _JSONDataclass:
+	'''A base class for dataclasses for conversion to JSON'''
+
+	def json(self):
+		'''Convert the object to a JSON string'''
+		out = dataclasses.asdict(self)
+		for key, value in out.items():
+			if isinstance(value, _JSONDataclass):
+				out[key] = value.json()
+		return json.dumps(out, default = _json_serialise_datetime)
+
+
+@dataclasses.dataclass
+class Item(_JSONDataclass):
 	'''An abstract base class for an item returned by the scraper's get_items generator.
 
 	An item can really be anything. The string representation should be useful for the CLI output (e.g. a direct URL for the item).'''
@@ -18,7 +42,8 @@ class Item:
 		pass
 
 
-class Entity:
+@dataclasses.dataclass
+class Entity(_JSONDataclass):
 	'''An abstract base class for an entity returned by the scraper's entity property.
 
 	An entity is typically the account of a person or organisation. The string representation should be the preferred direct URL to the entity's page on the network.'''