Compiling the docs with Sphinx 3.0.1 fails with AttributeError: 'Sphinx' object has no attribute 'info'

Are there plans to support building the docs with Sphinx 3?

```

# make -j5 -C doc html

make: Entering directory '/tmp/tmp.zp8i1A7t0T/pexpect/doc'

sphinx-build -b html -d _build/doctrees   . _build/html

Running Sphinx v3.0.1



Exception occurred:

  File "/tmp/tmp.zp8i1A7t0T/pexpect/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/sphinx-err-ezjlx78k.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:45: html] Error 2

make: Leaving directory '/tmp/tmp.zp8i1A7t0T/pexpect/doc'

```

Commenting out `app.info('Initializing GitHub plugin')` seems to make the build work.

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