update min python vesion in changelog

Change-Id: If35e5fb2faedc5172a0b20710616fa0f7fbb4bab

Author: CaselIT

doc/build/unreleased/py2.rst

@@ -1,4 +1,5 @@
 .. change::
     :tags: py3k
 
-    Remove support for Python 2. Mako now requires Python >= 3.6
+    Removed support for Python 2 and Python 3.6.
+    Mako now requires Python >= 3.7.