Switch Async HTTP-client to `httpx` (HTTP/2-support)

[thomasht86]

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• [x] Removed @staticmethod-decorator of callback_docv1 (See comment on bug below)
• [x] Switched to httpx - Mainly to add HTTP/2 support, which presumably could improve feed performance by a lot.
• [x] Unit tests passing ✅ - BUT, working to quantify performance gain.

⚠️ Encountered this bug when tenacity retry is applied

self = <AsyncRetrying object at 0x10468fca0 (stop=<tenacity.stop.stop_after_attempt object at 0x10468fb20>, wait=<tenacity.wa...bject at 0x10468fac0>, before=<function before_nothing at 0x104463f70>, after=<function after_nothing at 0x10447b1f0>)>
retry_state = <RetryCallState 4369142976: attempt #3; slept for 3.0; last result: failed (TypeError object dict can't be used in 'await' expression)>

    def iter(self, retry_state: "RetryCallState") -> t.Union[DoAttempt, DoSleep, t.Any]:  # noqa
        fut = retry_state.outcome
        if fut is None:
            if self.before is not None:
                self.before(retry_state)
            return DoAttempt()
    
        is_explicit_retry = fut.failed and isinstance(fut.exception(), TryAgain)
        if not (is_explicit_retry or self.retry(retry_state)):
            return fut.result()
    
        if self.after is not None:
            self.after(retry_state)
    
        self.statistics["delay_since_first_attempt"] = retry_state.seconds_since_start
        if self.stop(retry_state):
            if self.retry_error_callback:
>               return self.retry_error_callback(retry_state)
E               TypeError: 'staticmethod' object is not callable

notebooks/lib/python3.9/site-packages/tenacity/__init__.py:322: TypeError

We should make unit tests for error handling. Maybe this is part of your fix?