Merge pull request #396 from twisted/395-headers-tests #280

Job	Run time
docs	29s
Test 3.8 with lowest Twisted	1m 8s
lint	44s
Test 3.8 with latest Twisted	57s
Test 3.9 with lowest Twisted	50s
Test 3.9 with latest Twisted	51s
Test 3.10 with lowest Twisted	52s
Test 3.10 with latest Twisted	53s
Test pypy-3.9 with lowest Twisted	1m 46s
Test pypy-3.9 with latest Twisted	1m 49s
Test 3.11 with latest Twisted	50s
Test 3.12 with latest Twisted	1m 2s
Test pypy-3.10 with latest Twisted	1m 50s
Test 3.9 with trunk Twisted	58s
treq-required	1s
	15m 0s

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