readthedocs: fix requirements #786

Job	Run time
test-coverage	5m 28s
coverity-test	0s
build-test (ubuntu-20.04, gcc)	4m 39s
scanbuild	6m 45s
build-test (ubuntu-20.04, clang)	4m 39s
test-fuzz	2m 30s
build-test (fedora-32, gcc)	4m 50s
test-mbedtls (ubuntu-20.04)	3m 2s
test-no-crypto-build	1m 17s
build-test (fedora-32, clang)	5m 7s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 45s
build-test (opensuse-leap, gcc)	4m 45s
test-tcti-config	14m 32s
build-test (opensuse-leap, clang)	4m 47s
build-test (ubuntu-22.04, gcc)	4m 27s
build-test (ubuntu-22.04, clang)	4m 33s
build-test (alpine-3.15, gcc)	3m 59s
build-test (alpine-3.15, clang)	4m 57s
	1h 23m 2s

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