Add support for Python 3.8 and Python 3.9

Tests for those Python versions were added along in 032c4ed3 (ci: Switch
from Travis to GitHub Actions).

CHANGES.rst

@@ -8,6 +8,8 @@ Changelog
   that was the reason for its deprecation - has been understood and fixed. See
   (`#16 <https://github.com/zopefoundation/tempstorage/issues/16>`_).
 
+- Add support for Python 3.8 and Python 3.9.
+
 
 5.1 (2019-08-15)
 ----------------

setup.py

@@ -47,6 +47,8 @@ setup(name='tempstorage',
           "Programming Language :: Python :: 3.5",
           "Programming Language :: Python :: 3.6",
           "Programming Language :: Python :: 3.7",
+          "Programming Language :: Python :: 3.8",
+          "Programming Language :: Python :: 3.9",
           "Programming Language :: Python :: Implementation :: CPython",
           "Programming Language :: Python :: Implementation :: PyPy",
           "Topic :: Internet",