fix: scanny#746 update Python 3.x support in docs

Author: scanny

docs/user/install.rst

@@ -13,13 +13,13 @@ the Python Imaging Library (``PIL``). The charting features depend on
 satisfying these dependencies for you, but if you use the ``setup.py``
 installation method you will need to install the dependencies yourself.
 
-Currently |pp| requires Python 2.7, 3.3, 3.4, or 3.6. The tests are run against 2.7 and
-3.6 on Travis CI.
+Currently |pp| requires Python 2.7 or 3.3 or later. The tests are run against 2.7 and
+3.8 on Travis CI.
 
 Dependencies
 ------------
 
-* Python 2.6, 2.7, 3.3, 3.4, or 3.6
+* Python 2.6, 2.7, 3.3 or later
 * lxml
 * Pillow
 * XlsxWriter (to use charting features)