Autosummary fails to summarise docstrings that begin with overline section headings

[jakelishman]

Describe the bug

When using autosummary, it attempts to summarise paragraphs specially, and everything else it just takes the first line. This causes parsing errors if the first reST element in the source is a section heading with an overline, for example

=========
My Module
=========

It will take the first line (====) and insert it raw into a table cell, which results in invalid reST.

How to Reproduce

First unpack attached MWE project, which is a very minimal Python package, using autosummary to summarise two modules whose docstrings both begin with overline headings.

$ python -mpip install -e .
$ cd docs
$ make html

make html will show two warnings, and the generated HTML page (doc/_build/html/index.html) will have a table with empty entries in it, rather than showing text.

Expected behavior

The summariser should take the text of section headings as a summary.

Your project

MWE attached to reproduction steps.

Screenshots

The generated index.html file from the attached MWE. This is the incorrect behaviour: the table entries are empty because of parsing errors. (Ignore the Sphinx 3.5.4 in the screenshot - I accidentally took the screenshot using a build from an old venv, but the behaviour is present as of the latest 4.x.)

OS

Any

Python version

Any

Sphinx version

4.x branch

Sphinx extensions

sphinx.ext.autosummary

Extra tools

No response

Additional context

No response