Python 3.8 passes all tests

(cherry picked from commit 3c1005a)

Author: nedbat

CHANGES.rst

@@ -21,8 +21,8 @@ Unreleased
 - Namespace packages are supported on Python 3.7, where they used to cause
   TypeErrors about path being None. Fixes `issue 700`_.
 
-- Partial support for Python 3.8, which has not yet released an alpha. Fixes
-  `issue 707` and `issue 714`_.
+- Python 3.8 (as of today!) passes all tests.  Fixes `issue 707` and
+  `issue 714`_.
 
 - Development moved from `Bitbucket`_ to `GitHub`_.