Use built-in unittest for test execution

I like pytest, but I don't think it's necessary for super simple tests.
I'd like to avoid the dependency if we don't need it.

pyproject.toml

@@ -17,5 +17,4 @@ requires-python = ">=3.4"
 test = [
     "coverage",
     "flake8",
-    "pytest"
 ]

tox.ini

@@ -1,8 +1,3 @@
-# Tox (http://tox.testrun.org/) is a tool for running tests
-# in multiple virtualenvs. This configuration file will run the
-# test suite on all supported python versions. To use it, "pip install tox"
-# and then run "tox" from this directory.
-
 [tox]
 envlist = py34,py35,py36,37
 skipsdist = True
@@ -15,6 +10,6 @@ deps =
     flit
 commands =
     flit install
-    coverage run {envbindir}/pytest {posargs}
+    coverage run python -m unittest -s tests
     coverage report
     py37: flake8