fixing setup.py tests_require

Author: lockefox

setup.py

@@ -124,7 +124,7 @@ def run_tests(self):
         'retrying ~= 1.3.3'
     ],
     tests_require=[
-        'pytest~=3.1.0',
+        'pytest>=3.0.0',
         'pytest_cov~=2.4.0',        #requires requests==2.13.0
         'pytest-flask~=0.10.0',
         'pymysql~=0.7.10'