Fix tox for ci (#134)

* require virtualenv >= 20 in tox

* pin pip version for virtualenv creation

This is because later versions deprecate py27 and py35

* update tox version

Author: mdalp

.travis.yml

@@ -45,7 +45,7 @@ matrix:
       - TOX_ENV=py37-dj30-drf310
 
 install:
-  - travis_retry pip install "virtualenv~=14.0.0" "tox~=3.10.0" "coverage<4" "setuptools<40.0.0"
+  - travis_retry pip install "tox~=3.22.0" "coverage<4" "setuptools<40.0.0"
 
 script:
   - tox -ve $TOX_ENV

tox.ini

@@ -5,6 +5,7 @@ envlist =
     py{27,35,36,37}-dj{111}-drf{37,39}
     py{35,36,37}-dj{20,21,22}-drf{37,39}
     py{36,37}-dj{30}-drf{310}
+requires = virtualenv >= 20.0
 
 [pytest]
 norecursedirs = examples
@@ -13,6 +14,9 @@ flake8-max-line-length = 120
 
 [testenv]
 download = true
+setenv =
+    VIRTUALENV_PIP=20.2.3
+    VIRTUALENV_DOWNLOAD=1
 deps =
     -rrequirements/dev.txt
     dj19: Django~=1.9.12