Update the branch for tox3 git

Author: hroncok

tox.ini

@@ -10,7 +10,7 @@ extras =
 deps=
     tox315: tox >=3.15,<3.16
     toxrelease: tox < 4
-    toxmaster: git+https://github.com/tox-dev/tox.git@master
+    toxmaster: git+https://github.com/tox-dev/tox.git@legacy
 commands =
     pytest -v {posargs} tests