Fix middleware being patched multiple times when using FastAPI

[JohnnyDeuss]

When the StarletteIntegration is used in a FastAPI app, as instructed in the docs, the AuthenticationMiddleware and ExceptionMiddleware get patched every time add_middleware or add_exception_handler is called, since these methods call build_middleware_stack, where patched Middleware instances are repatched without checking if they were already patched.

If these functions are called enough times, Python registers this as infinite recursion (as we've discovered).

The changes look broad, but amount to the addition of two if not_yet_patched checks and indentation.

[antonpirker]

Hey @JohnnyDeuss
Can you fix the "Lint Sources" issues shown above?
You probably just need to reformat the code locally by calling black tests sentry_sdk in the root directory.