Eliminate use of `cgi` #241

Job	Run time
lint	57s
Test 3.8 with lowest Twisted	49s
docs	29s
Test 3.8 with latest Twisted	49s
Test 3.9 with lowest Twisted	50s
Test 3.9 with latest Twisted	45s
Test 3.10 with lowest Twisted	51s
Test 3.10 with latest Twisted	47s
Test pypy-3.9 with lowest Twisted	1m 32s
Test pypy-3.9 with latest Twisted	1m 33s
Test 3.7 with lowest Twisted	57s
Test pypy-3.8 with lowest Twisted	1m 32s
Test 3.11 with latest Twisted	48s
Test 3.12 with latest Twisted	1m 0s
Test pypy-3.10 with latest Twisted	1m 35s
Test 3.9 with trunk Twisted	50s
treq-required	1s
	16m 5s

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