readthedocs: downgrade sphinx #790

Job	Run time
scanbuild	6m 35s
test-coverage	5m 22s
build-test (ubuntu-20.04, gcc)	4m 38s
coverity-test	0s
build-test (ubuntu-20.04, clang)	4m 20s
test-fuzz	2m 34s
test-mbedtls (ubuntu-20.04)	2m 55s
build-test (fedora-32, gcc)	4m 38s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 39s
test-no-crypto-build	1m 16s
build-test (fedora-32, clang)	4m 59s
test-tcti-config	14m 12s
build-test (opensuse-leap, gcc)	4m 29s
build-test (opensuse-leap, clang)	4m 38s
build-test (ubuntu-22.04, gcc)	4m 25s
build-test (ubuntu-22.04, clang)	4m 47s
build-test (alpine-3.15, gcc)	3m 53s
build-test (alpine-3.15, clang)	4m 45s
	1h 21m 5s

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