readthedocs: downgrade sphinx #2267

Job	Run time
scanbuild	6m 35s
build-test (ubuntu-20.04, gcc)	4m 26s
test-mbedtls (ubuntu-20.04)	2m 45s
build-test (ubuntu-20.04, clang)	4m 22s
test-no-crypto-build	1m 16s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 38s
build-test (fedora-32, gcc)	4m 38s
build-test (fedora-32, clang)	5m 1s
build-test (opensuse-leap, gcc)	4m 30s
test-tcti-config	14m 21s
coverity-test	0s
build-test (opensuse-leap, clang)	4m 33s
build-test (ubuntu-22.04, gcc)	4m 26s
build-test (ubuntu-22.04, clang)	4m 29s
build-test (alpine-3.15, gcc)	4m 1s
test-coverage	5m 5s
build-test (alpine-3.15, clang)	4m 47s
test-fuzz	2m 28s
	1h 20m 21s

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