Typing cleanup

- Compatibility with older Python versions
  - use `Optional` and `Union` instead of `... | None` and `a | b`
  - use `typing_extensions.Protocol` instead of `typing.Protocol`
  - use `typing.Dict`, `typing.List`, etc. instead of the concrete types.

- Fix backend `.get()` annotations; not all were marked as `Optional[str]`
- Don't return anything from `Backend.set()` methods.
- The `Coder.decode_as_type()` type parameter must be a type to be
  compatible with `ModelField(..., type_=...)`.
- Clean up `Optional[]` use, remove where it is not needed.
- Clean up variable use in decorator, keeping the raw cached value
  separate from the return value from the wrapped endpoint.
- Annotate the wrapper as returning either the original type _or_ a
  Response (returning a 304 Not Modified response).
- Clean up small edge-case where `response` could be `None`.
- Correct type annotation on `JsonCoder.decode()` to match `Coder.decode()`.

tests/test_codecs.py

@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import Any, Optional
+from typing import Any, Optional, Tuple
 
 import pytest
 from pydantic import BaseModel, ValidationError
@@ -46,7 +46,7 @@ def test_pickle_coder(value: Any) -> None:
     [
         (1, None),
         ("some_string", None),
-        ((1, 2), tuple[int, int]),
+        ((1, 2), Tuple[int, int]),
         ([1, 2, 3], None),
         ({"some_key": 1, "other_key": 2}, None),
         (DCItem(name="foo", price=42.0, description="some dataclass item", tax=0.2), DCItem),