update travis config

Author: yusufcanb

.travis.yml

@@ -1,10 +1,16 @@
 language: python
-python:
-  - "3.6"
-  - "3.7"
-  - "3.8"
-  - "3.9"
+matrix:
+  include:
+    - python: 3.6
+      env: TOXENV=py36
+    - python: 3.7
+      env: TOXENV=py37
+    - python: 3.8
+      env: TOXENV=py38
+    - python: 3.9
+      env: TOXENV=py39
+
 install:
   - pip install tox
 script:
-  - tox .
+  - tox -e $TOXENV