Fix documentation building error for dnsdist and recursor

[rgacogne]

Short description

Fixes

The alabaster extension used by this project needs at least Sphinx v3.4; it therefore cannot be built with this version.

by pinning to the same version used by the auth.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 7446453056

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-5.5%) to 57.701%

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Files with Coverage Reduction	New Missed Lines	%
pdns/iputils.hh	1	73.93%
pdns/opensslsigners.cc	2	61.0%
modules/gpgsqlbackend/spgsql.cc	3	67.46%
pdns/rcpgenerator.cc	3	90.05%
modules/lmdbbackend/lmdbbackend.cc	14	72.5%

Totals
Change from base Build 7445955746:	-5.5%
Covered Lines:	107738
Relevant Lines:	155379

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💛 - Coveralls