relax httpx dep version and remove broken experimental BlockhashCache #834

Job	Run time
tests (tests/integration/test_async_http_client.py)	1m 18s
lint	1m 7s
tests (tests/integration/test_async_token_client.py)	1m 15s
tests (tests/integration/test_http_client.py)	1m 12s
tests (tests/integration/test_token_client.py)	1m 12s
tests (tests/integration/test_websockets.py)	2m 33s
tests (tests/integration/test_recent_performance_samples.py)	2m 2s
tests (tests/integration/test_memo.py)	1m 14s
tests (tests/unit)	29s
tests (src)	27s
test	1s
coverage	44s
	13m 34s

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