setup.py: Unpin pytest version but require >= 3.8.0

#36

setup.py

@@ -90,7 +90,7 @@ tests_require = [
     'moto',
     'pep8',
     'psutil',
-    'pytest == 3.6.4',
+    'pytest >= 3.8.0',
     'pytest-cov >= 2.6.0',
     'pytest-pep8',
     'pytest-pylint',