Update dependency tox to v4

setup.py

@@ -74,7 +74,7 @@ def run(self):
         'codecov>=2.0.3,<3.0.0',
         'coverage>=4.0.3,<5.0.0',
         'Sphinx>=1.4.1,<2.0.0',
-        'tox>=2.3.1,<3.0.0',
+        'tox>=4.14.1,<4.15.0',
         'virtualenv>=15.0.1,<16.0.0'
     ],
     url='https://github.com/PseudoSky/morphit',