Switch to ruff to handle linting and formatting

Ruff handles black, flake8 and isort in one package, and is way faster.
The isort rules had not been enforced, so this commit includes a lot
of import resorting changes.

I switched to flake8-bugbear and the standard black-compatible line
length of 80 + 10% (so max 88 characters), so some line reflowing is
included too.

Finally, because bugbear rightly points out that `setattr()` is less
performant, I've switched the `__signature__` assigment back to using
a direct assignment with type ignore comment.

examples/in_memory/main.py

@@ -4,13 +4,13 @@ from typing import Dict, Optional
 import pendulum
 import uvicorn
 from fastapi import FastAPI
+from pydantic import BaseModel
 from starlette.requests import Request
 from starlette.responses import JSONResponse, Response
 
 from fastapi_cache import FastAPICache
 from fastapi_cache.backends.inmemory import InMemoryBackend
 from fastapi_cache.decorator import cache
-from pydantic import BaseModel
 
 app = FastAPI()