bpo-37979: Add alternative to fromisoformat in documentation (GH-15596)

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
(cherry picked from commit 59725f3)

Co-authored-by: Paul Ganssle <[email protected]>

Author: miss-islington

Doc/library/datetime.rst

@@ -850,7 +850,9 @@ Other constructors, all class methods:
   .. 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
 

Misc/NEWS.d/next/Documentation/2019-08-29-10-40-05.bpo-37979.TAUx_E.rst

@@ -0,0 +1,2 @@
+Added a link to dateutil.parser.isoparse in the datetime.fromisoformat
+documentation. Patch by Paul Ganssle