Update tox envlist (#37)

- `unit` doesn't exist
- avoid running `func` tests by default
  (one may not wish to run func tests on their workstation,
  so it may be dangerous to run it by default)

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 skipsdist=True
-envlist = lint, unit, func
+envlist = lint
 skip_missing_interpreters = True
 
 [testenv]