Significantly overhaul configuration management

Everything now uses bog-standard Python dataclasses, with Pydantic
providing validation and type conversion through separate classes using
its type adapter feature. It's also possible to define your classes
using Pydantic's own model type directly, making the type adapter
unnecessary, but I didn't want to do things that way because no actual
validation is needed when constructing a Song instance for example.
Having Pydantic do its thing only on-demand was preferable.

I tried a number of validation libraries before settling on Pydantic for
this. It's not the fastest option out there (msgspec is I think), but it
makes adding support for third-party types like yarl.URL really easy, it
generates a nice clean JSON Schema which is easy enough to adjust to my
requirements through its GenerateJsonSchema hooks, and raw speed isn't
all that important anyway since this is a single-user desktop program
that reads its configuration file once on startup.

Also, MessagePack is now mandatory if you're caching to an external
service. It just didn't make a whole lot sense to explicitly install
mpd-now-playable's Redis or Memcached support and then use pickling with
them.

With all this fussing around done, I'm probably finally ready to
actually use that configuration file to configure new features! Yay!

src/mpd_now_playable/cache.py

@@ -1,37 +1,41 @@
 from __future__ import annotations
 
 from contextlib import suppress
-from typing import Any, Optional, TypeVar
+from typing import Any, Generic, Optional, TypeVar
 
 from aiocache import Cache
-from aiocache.serializers import BaseSerializer, PickleSerializer
+from aiocache.serializers import BaseSerializer
+from pydantic.type_adapter import TypeAdapter
 from yarl import URL
 
 T = TypeVar("T")
 
-HAS_ORMSGPACK = False
 with suppress(ImportError):
 	import ormsgpack
 
-	HAS_ORMSGPACK = True
 
-
-class OrmsgpackSerializer(BaseSerializer):
+class OrmsgpackSerializer(BaseSerializer, Generic[T]):
 	DEFAULT_ENCODING = None
 
-	def dumps(self, value: object) -> bytes:
-		return ormsgpack.packb(value)
+	def __init__(self, schema: TypeAdapter[T]):
+		super().__init__()
+		self.schema = schema
 
-	def loads(self, value: Optional[bytes]) -> object:
+	def dumps(self, value: T) -> bytes:
+		return ormsgpack.packb(self.schema.dump_python(value))
+
+	def loads(self, value: Optional[bytes]) -> T | None:
 		if value is None:
 			return None
-		return ormsgpack.unpackb(value)
+		data = ormsgpack.unpackb(value)
+		return self.schema.validate_python(data)
 
 
-def make_cache(url: URL, namespace: str = "") -> Cache[T]:
+def make_cache(schema: TypeAdapter[T], url: URL, namespace: str = "") -> Cache[T]:
 	backend = Cache.get_scheme_class(url.scheme)
 	if backend == Cache.MEMORY:
 		return Cache(backend)
+
 	kwargs: dict[str, Any] = dict(url.query)
 
 	if url.path:
@@ -48,6 +52,6 @@ def make_cache(url: URL, namespace: str = "") -> Cache[T]:
 
 	namespace = ":".join(s for s in [kwargs.pop("namespace", ""), namespace] if s)
 
-	serializer = OrmsgpackSerializer if HAS_ORMSGPACK else PickleSerializer
+	serializer = OrmsgpackSerializer(schema)
 
-	return Cache(backend, serializer=serializer(), namespace=namespace, **kwargs)
+	return Cache(backend, serializer=serializer, namespace=namespace, **kwargs)