Describe the bug
Following the 1st example from http://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html the code

:ref:`comparison manual <python:comparisons>`

creates a valid link. All good so far.

However, when I change the link to

:ref:`comparison manual <python:ipaddress>`

I get

WARNING: undefined label: python:ipaddress (if the link has no caption the label must precede a section header)

Searching in the objects.inv files according to the instructions in the link shows it should be a valid label

python -msphinx.ext.intersphinx https://docs.python.org/3/objects.inv | grep ipaddress
        ...
	ipaddress                                library/ipaddress.html#module-ipaddress
	howto/ipaddress                          An introduction to the ipaddress module : howto/ipaddress.html
	library/ipaddress                        ipaddress — IPv4/IPv6 manipulation library: library/ipaddress.html

To Reproduce
Steps to reproduce the behavior:

1. Follows the 1st example from http://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html
2. create the link :ref:comparison manual python:ipaddress`
3. generate docs e.g. sphinx-build -b html docs/source/ docs/build/

alternatively download the sample project attached and run make html

Expected behavior
I expect a valid link to be created to https://docs.python.org/3/library/ipaddress.html#module-ipaddress

Your project
broken-intersphinx.zip

Screenshots
n/a

Environment info

• OS: OSX 10.14.6
• Python version: 3.7.0
• Sphinx version: 2.2.0
• Sphinx extensions: intersphinx
• Extra tools: n/a

Additional context
n/a