Skip missing interpreters in tox.ini

tox.ini

@@ -4,7 +4,10 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py35, py36, py37, py38, doc
+envlist =
+    py{27,35,36,37,38,py,py3}, doc
+skip_missing_interpreters =
+    True
 
 [testenv]
 deps =
@@ -19,7 +22,8 @@ deps =
 # requires a Compiler and the build dependencies), you can download
 # it from http://www.lfd.uci.edu/~gohlke/pythonlibs/#lxml and install it via
 # $PYTHONDIR\Scripts\pip.exe install *.whl
-sitepackages=True
+sitepackages=
+    True
 commands =
     pip install -r requirements.txt -e ".[test]"
     py.test