tox: add pytestmaster and pytest60 environments

Author: bluetech

tox.ini

@@ -16,7 +16,9 @@ deps =
   pytest310: pytest~=3.10
   pytest46: pytest~=4.6
   pytest54: pytest~=5.4
+  pytest60: pytest~=6.0rc1
   pytestlatest: pytest
+  pytestmaster: git+https://github.com/pytest-dev/pytest.git@master
 platform=linux|darwin
 commands=
   pytest {posargs}