bug#37377: python-matplotlib-documentation build fails on master

[Maxim Cournoyer]

As of commit 83aa6562173e06e8fafd4857a2e708feabfe56f8:

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starting phase `build'
Running Sphinx v2.1.2
WARNING: while setting up extension matplotlib.sphinxext.mathmpl: directive 
'math' is already registered, it will be overridden
/gnu/store/2lc31q0b7kk21s6c1qln5gx6i090bbq5-python-sphinx-2.1.2/lib/python3.7/site-packages/sphinx/util/docutils.py:311:
 RemovedInSphinx30Warning: function based directive support is now deprecated. 
Use class based directive instead.
  RemovedInSphinx30Warning)
/gnu/store/2lc31q0b7kk21s6c1qln5gx6i090bbq5-python-sphinx-2.1.2/lib/python3.7/site-packages/sphinx/util/docutils.py:311:
 RemovedInSphinx30Warning: function based directive support is now deprecated. 
Use class based directive instead.
  RemovedInSphinx30Warning)

Exception occurred:
  File 
"/tmp/guix-build-python-matplotlib-documentation-2.2.3.drv-0/matplotlib-2.2.3/doc/sphinxext/github.py",
 line 149, in setup
    app.info('Initializing GitHub plugin')
AttributeError: 'Sphinx' object has no attribute 'info'
The full traceback has been saved in 
/tmp/guix-build-python-matplotlib-documentation-2.2.3.drv-0/sphinx-err-n9mt2cr6.log,
 if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message 
can be provided next time.
A bug report can be filed in the tracker at 
<https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [Makefile:20: html] Error 2
Backtrace:
           4 (primitive-load "/gnu/store/ksj56pxrr1plazg6nrppi1r4hz6…")
In ice-9/eval.scm:
   191:35  3 (_ _)
In srfi/srfi-1.scm:
   863:16  2 (every1 #<procedure 71f240 at /gnu/store/4r04fsfcryy5h…> …)
In 
/gnu/store/4r04fsfcryy5h4v2h3g6lzlibafmmdjm-module-import/guix/build/gnu-build-system.scm:
   799:28  1 (_ _)
In 
/gnu/store/4r04fsfcryy5h4v2h3g6lzlibafmmdjm-module-import/guix/build/utils.scm:
    616:6  0 (invoke _ . _)

/gnu/store/4r04fsfcryy5h4v2h3g6lzlibafmmdjm-module-import/guix/build/utils.scm:616:6:
 In procedure invoke:
Throw to key `srfi-34' with args `(#<condition &invoke-error [program: "make" 
arguments: ("SPHINXBUILD=sphinx-build" "SPHINXOPTS=" "html" "texinfo") 
exit-status: 2 term-signal: #f stop-signal: #f] 8a9180>)'.
note: keeping build directory 
`/tmp/guix-build-python-matplotlib-documentation-2.2.3.drv-1'
builder for 
`/gnu/store/dwrp4g4ajqd8szxr93id0pwqimf0bafg-python-matplotlib-documentation-2.2.3.drv'
 failed with exit code 1
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