docs: change some last 3.6 references to 3.7 with dropping of 3.6.

Author: rouilj

doc/features.txt

@@ -18,7 +18,7 @@ from Ka-Ping Yee in the :index:`Software Carpentry` "Track" design competition.
   ``python demo.py`` or `a Docker container`_ in under 5 minutes. 
 - Choose from `five included templates`_ for your tracker.
 - `Customize`_ and use the demo as a template for your production tracker.
-- No need for additional support software, just Python (3.6+ or 2.7)
+- No need for additional support software, just Python (3.7+ or 2.7)
   to get started.
 - Basic installation (including a web interface) takes about 30 minutes.
 - Supports embedded databases like `SQLite`_ and dbm. Upgradable

doc/rest.txt

@@ -375,8 +375,8 @@ The rest interface returns status 406 if you use an unrecognized
 extension.  You will also get a 406 status if none of the entries in
 the accept header are available or if the accept header is invalid.
 
-Note: ``dicttoxml2.py`` is an updated version of ``dicttoxml.py``. If
-you are still using Python 2.7 or 3.6, you can use ``dicttoxml.py``.
+Note: ``dicttoxml2.py`` is an updated version of ``dicttoxml.py`` and
+should be used for Roundup running on Python 3.7 or newer.
 
 Also the ``/binary_content`` attribute endpoint can be used to
 retrieve raw file data in many formats.