Improve tox.ini

tox.ini

@@ -1,8 +1,15 @@
 [tox]
 envlist =
-    py{36,37,38,39}-django{22,30,31,trunk}
+    py{36,37,38,39}-django{22,30,31,master}
     flake8
 
+[travis]
+python =
+    3.6: py36
+    3.7: py37
+    3.8: py38
+    3.9: py39
+
 [testenv]
 deps =
     freezegun==0.3.12
@@ -10,7 +17,7 @@ deps =
     django22: Django==2.2.*
     django30: Django==3.0.*
     django31: Django==3.1.*
-    djangotrunk: https://github.com/django/django/archive/master.tar.gz
+    djangomaster: https://github.com/django/django/archive/master.tar.gz
 ignore_outcome =
     djangotrunk: True
 passenv =
@@ -20,13 +27,17 @@ passenv =
 
 commands =
     pip install -e .
-    py.test {posargs}
+    pytest {posargs}
+
+[testenv:py{36,37,38,39}-djangomaster]
+ignore_errors = True
 
 [testenv:flake8]
 basepython =
     python3.7
 deps =
     flake8
+skip_install = True
 commands =
     flake8 model_utils tests