readthedocs: downgrade sphinx #2264

Job	Run time
scanbuild	6m 35s
coverity-test	0s
build-test (ubuntu-20.04, gcc)	4m 34s
build-test (ubuntu-20.04, clang)	4m 26s
test-coverage	5m 16s
test-mbedtls (ubuntu-20.04)	2m 49s
build-test (fedora-32, gcc)	4m 54s
test-fuzz	2m 34s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 39s
build-test (fedora-32, clang)	5m 3s
test-no-crypto-build	1m 21s
build-test (opensuse-leap, gcc)	4m 41s
test-tcti-config	14m 31s
build-test (opensuse-leap, clang)	4m 43s
build-test (ubuntu-22.04, gcc)	4m 27s
build-test (ubuntu-22.04, clang)	4m 45s
build-test (alpine-3.15, gcc)	4m 1s
build-test (alpine-3.15, clang)	4m 55s
	1h 22m 14s

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