sphinx 2.0.0-3.2.0 doesn't install texinfo images

[oerdnj]

Describe the bug

The sphinx-build command (since version 2.0.0) doesn't install images into build directory even when it claims it is doing so.

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/FRRouting/frr.git
$ cd frr
$ sphinx-build -b texinfo -d "doc/user/_build/.doctrees" "doc/user" "doc/user/_build/texinfo"
Running Sphinx v3.2.0
making output directory... done
/home/ondrej/Projects/pkg-frr/frr-7.4/doc/user/conf.py:368: RemovedInSphinx40Warning: The app.add_stylesheet() is deprecated. Please use app.add_css_file() instead.
  app.add_stylesheet('overrides.css')
/home/ondrej/Projects/pkg-frr/frr-7.4/doc/user/conf.py:369: RemovedInSphinx40Warning: The app.add_javascript() is deprecated. Please use app.add_js_file() instead.
  app.add_javascript('overrides.js')
building [mo]: targets for 0 po files that are out of date
building [texinfo]: all documents
updating environment: [new config] 35 added, 0 changed, 0 removed
reading sources... [100%] zebra
[...some duplicate clicmd description warnings...]
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
processing frr.texi... index overview installation setup basic vtysh filter routemap ipv6 kernel snmp zebra bfd bgp babeld fabricd ldpd eigrpd isisd nhrpd ospfd ospf6d pim pbr ripd ripngd sharp static vnc vrrp bmp watchfrr bugs packet-dumps glossary
resolving references...
done
writing... copying images... [ 11%] ../figures/fig-normal-processing.png                                                                                                                                               copying images... [100%] ../figures/fig-vnc-redundant-route-reflectors.png
done
copying Texinfo support files... Makefile done
build succeeded, 56 warnings.

The Texinfo files are in doc/user/_build/texinfo.
Run 'make' in that directory to run these through makeinfo
(use 'make info' here to do that automatically).
$ find doc/user/_build/texinfo -type f -name \*.png
# nothing, no images found
$

Expected behavior
The images should be copied (this is output from Python 3.7.3 + sphinx-build 1.8.4):

$ sphinx-build -b texinfo -d "doc/user/_build/.doctrees" "doc/user" "doc/user/_build/texinfo"
Running Sphinx v1.8.4
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [texinfo]: all documents
updating environment: 0 added, 0 changed, 0 removed
looking for now-outdated files... none found
processing frr.texi...index overview installation setup basic vtysh filter routemap ipv6 kernel snmp zebra bfd bgp babeld fabricd ldpd eigrpd isisd nhrpd ospfd ospf6d pim pbr ripd ripngd sharp static vnc vrrp bmp watchfrr bugs packet-dumps glossary
resolving references...
writing... done
copying images... [100%] ../figures/fig-vnc-redundant-route-reflectors.png
copying Texinfo support files... /build/frr-7.4/doc/user/_build/texinfo/Makefile done
build succeeded.
$ find doc/user/_build/texinfo -type f -name \*.png
The Texinfo files are in doc/user/_build/texinfo.
Run 'make' in that directory to run these through makeinfo
(use 'make info' here to do that automatically).
doc/user/_build/texinfo/fig-vnc-mesh.png
doc/user/_build/texinfo/fig-vnc-frr-route-reflector.png
doc/user/_build/texinfo/fig_topologies_full.png
doc/user/_build/texinfo/fig-vnc-redundant-route-reflectors.png
doc/user/_build/texinfo/frr-figures/ospf_api_architecture.png
doc/user/_build/texinfo/frr-figures/fig-vnc-mesh.png
doc/user/_build/texinfo/frr-figures/fig-vnc-frr-route-reflector.png
doc/user/_build/texinfo/frr-figures/ospf_api_msgs1.png
doc/user/_build/texinfo/frr-figures/threadmaster-multiple.png
doc/user/_build/texinfo/frr-figures/frr-logo-small.png
doc/user/_build/texinfo/frr-figures/fig_topologies_full.png
doc/user/_build/texinfo/frr-figures/ospf_api_msghdr.png
doc/user/_build/texinfo/frr-figures/fig-vnc-gw-rr.png
doc/user/_build/texinfo/frr-figures/fig-vnc-redundant-route-reflectors.png
doc/user/_build/texinfo/frr-figures/frr-logo-medium.png
doc/user/_build/texinfo/frr-figures/fig-rs-processing.png
doc/user/_build/texinfo/frr-figures/fig-normal-processing.png
doc/user/_build/texinfo/frr-figures/frr-logo-icon.png
doc/user/_build/texinfo/frr-figures/fig_topologies_rs.png
doc/user/_build/texinfo/frr-figures/frr-logo.png
doc/user/_build/texinfo/frr-figures/git_branches.png
doc/user/_build/texinfo/frr-figures/cligraph.png
doc/user/_build/texinfo/frr-figures/threadmaster-single.png
doc/user/_build/texinfo/frr-figures/ospf_api_msgs2.png
doc/user/_build/texinfo/frr-figures/fig-vnc-commercial-route-reflector.png
doc/user/_build/texinfo/frr-figures/fig-vnc-gw.png
doc/user/_build/texinfo/fig-rs-processing.png
doc/user/_build/texinfo/fig-normal-processing.png
doc/user/_build/texinfo/fig_topologies_rs.png
doc/user/_build/texinfo/fig-vnc-commercial-route-reflector.png
doc/user/_build/texinfo/fig-vnc-gw.png

Your project
https://github.com/FRRouting/frr/blob/master/doc/user/conf.py

Environment info

• OS: Debian unstable
• Python version: 3.8.5
• Sphinx version: 3.2.0 (both packaged and installed via pip)

Additional context
Copying images works with other output formats (checked singlehtml). I can confirm (via trying various versions from pypi) that this is regression between 1.8.5 and 2.0.0 (the 2.0.0 is a first version that doesn't copy the files).