relax httpx dep version and remove broken experimental BlockhashCache #837

Job	Run time
lint	1m 11s
tests (tests/integration/test_async_http_client.py)	1m 25s
tests (tests/integration/test_async_token_client.py)	1m 30s
tests (tests/integration/test_http_client.py)	1m 32s
tests (tests/integration/test_token_client.py)	1m 32s
tests (tests/integration/test_websockets.py)	2m 42s
tests (tests/integration/test_recent_performance_samples.py)	2m 16s
tests (tests/integration/test_memo.py)	1m 31s
tests (tests/unit)	1m 1s
tests (src)	54s
test	1s
coverage	28s
	16m 3s

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