Add Python 3.7, 3.8, and 3.9 to tox configuration

Author: Harmon758

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = flake8, py34, py35, py36
+envlist = flake8, py34, py35, py36, py37, py38, py39
 
 [testenv]
 commands = nosetests {posargs:--with-coverage --cover-package=smmap}