docs: remove line about Python 2.7 being unsupported (#808)

* docs: remove line about Python 2.7 being unsupported

According to the README and setup.py, we do support Python 2.7.

Also clarify that the utils module and its positional decorator
do still exist and does work in both Py2 and Py3

* Update migrating.rst

Author: Bob "Wombat" Hogg

packages/google-cloud-ndb/docs/migrating.rst

@@ -174,22 +174,21 @@ a number of methods that were mostly used internally. Some of those have been
 made obsolete by new Python 3 features, while others have been discarded due
 to implementation differences in the new `ndb`.
 
-Possibly the most used utility from this module outside of `ndb` code, is the
+Possibly the most used utility from this module outside of `ndb` code is the
 ``positional`` decorator, which declares that only the first `n` arguments of
 a function or method may be positional. Python 3 can do this using keyword-only
 arguments. What used to be written as::
 
     @utils.positional(2)
-    def function1(arg1, arg2, arg3=None, arg4=None)
+    def function1(arg1, arg2, arg3=None, arg4=None):
         pass
 
-Will be written like this in the new version::
+Should be written like this in Python 3::
 
-    def function1(arg1, arg2, *, arg3=None, arg4=None)
+    def function1(arg1, arg2, *, arg3=None, arg4=None):
         pass
 
-Note that this could change if Python 2.7 support is added at some point, which
-is still a possibility.
+However, ``positional`` remains available and works in both Python 2 and 3.
 
 Exceptions
 ==========