chore: using TOX_ENV to specify tox python version

.travis.yml

@@ -10,23 +10,29 @@ addons:
 jobs:
   include:
     - python: 3.6
+      env: TOX_ENV=py36-full
     - python: 3.7
+      env: TOX_ENV=py37-full
     - python: 3.8
+      env: TOX_ENV=py38-full
     - python: 3.9
+      env: TOX_ENV=py39-full
     - os: osx
+      osx_image: xcode12.2
       language: shell
+      env: TOX_ENV=py38-full
     - os: windows
       language: shell
       before_install:
-        - choco install python --version 3.8.0
+        - choco install python --version 3.8.5
         - python -m pip install --upgrade pip
-      env: PATH=/c/Python38:/c/Python38/Scripts:$PATH
+      env: PATH=/c/Python38:/c/Python38/Scripts:$PATH:TOX_ENV=py38-full
 
 install:
-  - pip install tox-travis codecov
+  - pip3 install tox codecov
 
 script:
-  - tox
+  - tox -e $TOX_ENV
 
 after_success:
   - codecov