Fix ARM build on readthedocs.org

By default readthedocs.org uses Sphinx 1.8.5, but MR !4563 has
introduced depedency on ReferenceRole class which is available only in
Sphinx 2.0.0.

Path to doc/arm/requirements.txt needs to be configured in
readthedocs.org.
70 jobs for !4968 with pspacek/rtd-sphinx-fix in 17 seconds (queued for 10 seconds)
latest detached
Status Job ID Name Coverage
  Autoconf
canceled #1682865
amd64 linux
autoreconf

00:00:16

 
  Precheck
canceled #1682867
amd64 linux
clang-format

00:00:16

canceled #1682868
amd64 linux
coccinelle

00:00:15

canceled #1682869
amd64 linux
danger

00:00:15

canceled #1682866
amd64 linux
misc

00:00:16

canceled #1682870
amd64 linux
tarball-create

 
  Build
canceled #1682881
amd64 linux
clang:asan

canceled #1682885
amd64 linux
clang:buster:amd64

canceled #1682888
amd64 libvirt
clang:freebsd11:amd64

canceled #1682889
amd64 libvirt
clang:freebsd12:amd64

canceled #1682890
amd64 libvirt
clang:openbsd:amd64

canceled #1682883
amd64 linux
clang:tsan

canceled #1682871
amd64 linux
gcc:alpine3.13:amd64

canceled #1682880
amd64 linux
gcc:asan

canceled #1682875
amd64 linux
gcc:buster:amd64

canceled #1682872
amd64 linux
gcc:centos7:amd64

canceled #1682873
amd64 linux
gcc:centos8:amd64

canceled #1682879
amd64 linux
gcc:focal:amd64

canceled #1682884
amd64 linux
gcc:mutexatomics

canceled #1682877
amd64 linux
gcc:out-of-tree

canceled #1682876
amd64 linux
gcc:sid:amd64

canceled #1682886
amd64 linux
gcc:softhsm2.4

canceled #1682887
amd64 linux
gcc:softhsm2.6

canceled #1682874
amd64 linux
gcc:stretch:amd64

canceled #1682882
amd64 linux
gcc:tsan

canceled #1682878
amd64 linux
gcc:tumbleweed:amd64

canceled #1682891
amd64 windows
msvc:windows:amd64

00:00:13

 
  Unit
canceled #1682901
amd64 linux
unit:clang:asan

canceled #1682905
amd64 linux
unit:clang:buster:amd64

canceled #1682908
amd64 libvirt
unit:clang:freebsd11:amd64

canceled #1682909
amd64 libvirt
unit:clang:freebsd12:amd64

canceled #1682910
amd64 libvirt
unit:clang:openbsd:amd64

canceled #1682903
amd64 linux
unit:clang:tsan

canceled #1682892
amd64 linux
unit:gcc:alpine3.13:amd64

canceled #1682900
amd64 linux
unit:gcc:asan

canceled #1682896
amd64 linux
unit:gcc:buster:amd64

canceled #1682893
amd64 linux
unit:gcc:centos7:amd64

canceled #1682894
amd64 linux
unit:gcc:centos8:amd64

canceled #1682899
amd64 linux
unit:gcc:focal:amd64

canceled #1682904
amd64 linux
unit:gcc:mutexatomics

canceled #1682897
amd64 linux
unit:gcc:sid:amd64

canceled #1682906
amd64 linux
unit:gcc:softhsm2.4

canceled #1682907
amd64 linux
unit:gcc:softhsm2.6

canceled #1682895
amd64 linux
unit:gcc:stretch:amd64

canceled #1682902
amd64 linux
unit:gcc:tsan

canceled #1682898
amd64 linux
unit:gcc:tumbleweed:amd64

 
  System
canceled #1682920
amd64 linux
system:clang:asan

canceled #1682924
amd64 linux
system:clang:buster:amd64

canceled #1682927
amd64 libvirt
system:clang:freebsd11:amd64

canceled #1682928
amd64 libvirt
system:clang:freebsd12:amd64

canceled #1682922
amd64 linux
system:clang:tsan

canceled #1682911
amd64 linux
system:gcc:alpine3.13:amd64

canceled #1682919
amd64 linux
system:gcc:asan

canceled #1682915
amd64 linux
system:gcc:buster:amd64

canceled #1682912
amd64 linux
system:gcc:centos7:amd64

canceled #1682913
amd64 linux
system:gcc:centos8:amd64

canceled #1682918
amd64 linux
system:gcc:focal:amd64

canceled #1682923
amd64 linux
system:gcc:mutexatomics

canceled #1682916
amd64 linux
system:gcc:sid:amd64

canceled #1682925
amd64 linux
system:gcc:softhsm2.4

canceled #1682926
amd64 linux
system:gcc:softhsm2.6

canceled #1682914
amd64 linux
system:gcc:stretch:amd64

canceled #1682921
amd64 linux
system:gcc:tsan

canceled #1682917
amd64 linux
system:gcc:tumbleweed:amd64

 
  Docs
canceled #1682929
amd64 linux
docs

 
  Postcheck
canceled #1682933
amd64 linux
cppcheck

canceled #1682930
amd64 linux
flake8

canceled #1682934
amd64 linux
gcov

canceled #1682931
amd64 linux
pylint

canceled #1682932
amd64 linux
scan-build