Adds a link to `dateutil.parser.isoparse` in the documentation.
It would be nice to set up intersphinx for things like this, but I think we can leave that for a separate PR.
CC: @pitrou
[bpo-37979](https://bugs.python.org/issue37979)
https://bugs.python.org/issue37979
Automerge-Triggered-By: @pitrou
.. caution::
This does not support parsing arbitrary ISO 8601 strings - it is only intended
- as the inverse operation of :meth:`datetime.isoformat`.
+ as the inverse operation of :meth:`datetime.isoformat`. A more full-featured
+ ISO 8601 parser, ``dateutil.parser.isoparse`` is available in the third-party package
+ `dateutil <https://dateutil.readthedocs.io/en/stable/parser.html#dateutil.parser.isoparse>`_.
.. versionadded:: 3.7
--- /dev/null
+Added a link to dateutil.parser.isoparse in the datetime.fromisoformat
+documentation. Patch by Paul Ganssle