remove idna as a primary dependency (#4624)

* remove idna as a primary dependency

* empty commit

* dynamodb test fix (thanks to Matt Bullock)

* review feedback
diff --git a/docs/faq.rst b/docs/faq.rst
index 409f6ce..dce94b7 100644
--- a/docs/faq.rst
+++ b/docs/faq.rst
@@ -99,6 +99,18 @@
 with any version of Python greater than or equal to 3.4. Recent versions of
 ``pip`` will automatically install ``abi3`` wheels.
 
+``ImportError``: ``idna`` is not installed
+------------------------------------------
+
+``cryptography`` deprecated passing :term:`U-label` strings to various X.509
+constructors in version 2.1 and in version 2.5 moved the ``idna`` dependency
+to a ``setuptools`` extra. If you see this exception you should upgrade your
+software so that it no longer depends on this deprecated feature. If that is
+not yet possible you  can also install ``cryptography`` with
+``pip install cryptography[idna]`` to automatically install the missing
+dependency. This workaround will be available until the feature is fully
+removed.
+
 .. _`NaCl`: https://nacl.cr.yp.to/
 .. _`PyNaCl`: https://pynacl.readthedocs.io
 .. _`WSGIApplicationGroup`: https://modwsgi.readthedocs.io/en/develop/configuration-directives/WSGIApplicationGroup.html