Mention 3.6 instead of 3.5 in docs (xonsh#5193)

* Update intro.rst to mention Python 3.6

The homepage mentions 3.6, so I update this section to be in line

* Create upgrade-python-version.rst

Add news item for xonsh#5193

docs/intro.rst

@@ -54,7 +54,7 @@
     </p>
 
 Xonsh is a Python-powered, cross-platform, Unix-gazing shell language and
-command prompt. The language is a superset of Python 3.5+ with additional
+command prompt. The language is a superset of Python 3.6+ with additional
 shell primitives that you are used to from Bash and IPython. It works on
 all major systems including Linux, Mac OSX, and Windows. Xonsh is meant
 for the daily use of experts and novices alike.

news/upgrade-python-version.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* Upgrade Python version at the introduction (https://github.com/xonsh/xonsh/pull/5193)
+
+**Security:**
+
+* <news item>