Cut BaseProtocol circular reference on close. #483

Job	Run time
test-postgres (9.5)	3m 17s
test-platforms (3.8, ubuntu-latest, asyncio)	2m 37s
test-postgres (9.6)	3m 49s
test-platforms (3.8, ubuntu-latest, uvloop)	2m 16s
test-postgres (10)	2m 59s
test-platforms (3.8, macos-latest, asyncio)	3m 2s
test-platforms (3.8, macos-latest, uvloop)	2m 10s
test-postgres (11)	2m 59s
test-platforms (3.8, windows-latest, asyncio)	3m 50s
test-postgres (12)	3m 27s
test-postgres (13)	3m 40s
test-platforms (3.9, ubuntu-latest, asyncio)	2m 17s
test-postgres (14)	3m 11s
test-platforms (3.9, ubuntu-latest, uvloop)	2m 32s
test-postgres (15)	3m 31s
test-platforms (3.9, macos-latest, asyncio)	1m 20s
test-postgres (16)	3m 30s
test-platforms (3.9, macos-latest, uvloop)	1m 14s
test-platforms (3.9, windows-latest, asyncio)	3m 38s
test-platforms (3.10, ubuntu-latest, asyncio)	2m 15s
test-platforms (3.10, ubuntu-latest, uvloop)	2m 19s
test-platforms (3.10, macos-latest, asyncio)	3m 16s
test-platforms (3.10, macos-latest, uvloop)	2m 55s
test-platforms (3.10, windows-latest, asyncio)	9m 10s
test-platforms (3.11, ubuntu-latest, asyncio)	2m 3s
test-platforms (3.11, ubuntu-latest, uvloop)	2m 9s
test-platforms (3.11, macos-latest, asyncio)	3m 31s
test-platforms (3.11, macos-latest, uvloop)	2m 42s
test-platforms (3.11, windows-latest, asyncio)	7m 5s
test-platforms (3.12, ubuntu-latest, asyncio)	2m 54s
test-platforms (3.12, macos-latest, asyncio)	3m 58s
test-platforms (3.12, windows-latest, asyncio)	5m 8s
Regression Tests	0s
	1h 44m 44s

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