build(deps): bump sphinx-rtd-theme from 2.0.0 to 3.0.1 in /lib/spack/docs #728

Job	Run time
changes	4s
prechecks / style	58s
prechecks / validate	39s
prechecks / bootstrap-dev-rhel8	4m 19s
prechecks / audit / package-audits (windows-latest, powershell Invoke-Expression -Command "./share/spack/qa/windows_t...	2m 44s
prechecks / audit / package-audits (ubuntu-latest, bash)	4m 19s
prechecks / audit / package-audits (macos-latest, bash)	3m 51s
all-prechecks	1s
bootstrap	0s
unit-tests	0s
all	0s
	16m 55s

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