update tox to run unit tests

tox.ini

@@ -2,18 +2,14 @@
 envlist = py37,py38,py39
 minversion = 3.7
 skipsdist = True
+skip_missing_interpreters = true
 
 [testenv]
 skip_install = True
-deps =
-    -e
-    .[test]
-commands = pytest {posargs:rocksdb/tests}
+deps = -e .
+commands = python -m unittest discover . -v
+norecursedirs = .tox
 
 [testenv:docs]
 deps = .[doc]
 commands = python setup.py build_sphinx -W
-
-[pytest]
-addopts = --verbose
-norecursedirs = .tox