Improve async body iter #585

Job	Run time
package	24s
macOS 3.7	1s
Windows 3.7	1s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.7	1s
macOS 3.8	1s
Windows 3.8	1s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.8	1s
macOS 3.9	1s
Windows 3.9	1s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.9	1s
Ubuntu 22 (OpenSSL 3+) 3.9	1s
macOS 3.10	2m 5s
Windows 3.10	1m 58s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.10	2m 6s
Ubuntu 22 (OpenSSL 3+) 3.10	2m 5s
macOS 3.11	2m 6s
Windows 3.11	1m 50s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.11	2m 5s
Ubuntu 22 (OpenSSL 3+) 3.11	2m 4s
macOS 3.12	1m 48s
Windows 3.12	19s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.12	1m 36s
Ubuntu 22 (OpenSSL 3+) 3.12	56s
macOS 3.13	8s
Windows 3.13	1s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.13	1s
Ubuntu 22 (OpenSSL 3+) 3.13	1s
Ubuntu 22 (OpenSSL 3+) 3.14	1s
Ubuntu 22 (OpenSSL 3+) pypy-3.7 test-pypy	1s
Ubuntu 22 (OpenSSL 3+) pypy-3.8 test-pypy	1s
Ubuntu 22 (OpenSSL 3+) pypy-3.9-v7.3.13 test-pypy	1s
Ubuntu 22 (OpenSSL 3+) pypy-3.10 test-pypy	1s
Ubuntu 22 (OpenSSL 3+) 3.x test_brotlipy	1s
Ubuntu 20.04 (OpenSSL 1.1.1) 3.9.2 test-3.9	1s
coverage	7s
	21m 57s

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