Cut BaseProtocol circular reference on close. #427

Job	Run time
test-postgres (9.5)	3m 12s
test-postgres (9.6)	3m 24s
test-postgres (10)	2m 38s
test-postgres (11)	2m 41s
test-postgres (12)	2m 37s
test-postgres (13)	2m 36s
test-postgres (14)	2m 51s
test-postgres (15)	2m 54s
test-platforms (3.7, ubuntu-latest, asyncio)	2m 40s
test-platforms (3.7, ubuntu-latest, uvloop)	2m 30s
test-platforms (3.7, macos-latest, asyncio)	2m 55s
test-platforms (3.7, macos-latest, uvloop)	2m 10s
test-platforms (3.7, windows-latest, asyncio)	5m 6s
test-platforms (3.8, ubuntu-latest, asyncio)	2m 35s
test-platforms (3.8, ubuntu-latest, uvloop)	2m 13s
test-platforms (3.8, macos-latest, asyncio)	2m 7s
test-platforms (3.8, macos-latest, uvloop)	1m 54s
test-platforms (3.8, windows-latest, asyncio)	5m 7s
test-platforms (3.9, ubuntu-latest, asyncio)	2m 14s
test-platforms (3.9, ubuntu-latest, uvloop)	2m 23s
test-platforms (3.9, macos-latest, asyncio)	1m 27s
test-platforms (3.9, macos-latest, uvloop)	1s
test-platforms (3.9, windows-latest, asyncio)	5m 7s
test-platforms (3.10, ubuntu-latest, asyncio)	2m 30s
test-platforms (3.10, ubuntu-latest, uvloop)	2m 12s
test-platforms (3.10, macos-latest, asyncio)	2m 49s
test-platforms (3.10, macos-latest, uvloop)	3m 5s
test-platforms (3.10, windows-latest, asyncio)	4m 54s
test-platforms (3.11, ubuntu-latest, asyncio)	2m 0s
test-platforms (3.11, ubuntu-latest, uvloop)	2m 24s
test-platforms (3.11, macos-latest, asyncio)	3m 33s
test-platforms (3.11, macos-latest, uvloop)	5m 10s
test-platforms (3.11, windows-latest, asyncio)	5m 6s
Regression Tests	0s
	1h 37m 5s

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