Upgrade to solders 0.20 #827

Job	Run time
tests (tests/integration/test_async_http_client.py)	1m 16s
tests (tests/integration/test_async_http_client.py)	1m 16s
lint	1m 12s
lint	1m 12s
tests (tests/integration/test_async_token_client.py)	1m 8s
tests (tests/integration/test_async_token_client.py)	1m 8s
tests (tests/integration/test_http_client.py)	1m 9s
tests (tests/integration/test_http_client.py)	1m 14s
tests (tests/integration/test_token_client.py)	1m 12s
tests (tests/integration/test_token_client.py)	1m 12s
tests (tests/integration/test_websockets.py)	2m 20s
tests (tests/integration/test_websockets.py)	2m 20s
tests (tests/integration/test_recent_performance_samples.py)	2m 10s
tests (tests/integration/test_recent_performance_samples.py)	2m 10s
tests (tests/integration/test_memo.py)	1m 10s
tests (tests/integration/test_memo.py)	1m 10s
tests (tests/unit)	38s
tests (tests/unit)	38s
tests (src)	28s
tests (src)	28s
test	0s
test	1s
coverage	0s
coverage	38s
	26m 10s

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