Removed an unnecessary TODO message and reverted a class name change for BaseHTTPTransport.

GH-4568

Adds test coverage for the error handling path when HTTP requests return
error status codes.

GH-4568

Restore comments accidentally removed during a previous commit.

Refactored class names such that BaseHttpTransport now has the same functionality as before the hierarchy refactor

GH-4568

Add a new flush_async method in the Transport ABC. This is needed for the async transport, as calling it from the client
while preserving execution order in close will require flush to be a coroutine, not a function.

GH-4568

Move flush_async down to the specific async transport subclass. This makes more sense anyway, as
this will only be required by the async transport. If more async transports are expected,
another shared superclass can be created.

GH-4568

Add necessary type annotations to the core HttpTransport to accomodate for async transport.

GH-4568

…nd cleaner return paths

GH-4568

GH-4568

Add a new implementation of the transport background worker based on an
async task. This worker mostly mirrors the same functionality as the
thread-based worker, with the exception that it exposes a non-blocking
async flush (which can be awaited from an async context). Furthermore,
the worker itself is not thread-safe and should be called using
[run_coroutine_threadsafe](https://docs.python.org/3/library/asyncio-task.html#asyncio.run_coroutine_threadsafe)
or similar when called from another thread (this is fixed and handled by
the transport). I have kept the fork check from the threaded worker, but
I am not sure if it is necessary as forking in an async application
would also break the event loop.

GH-4581

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Co-authored-by: Neel Shah <neel.shah@sentry.io>