add 3.10 to travis config too

Author: hjwp

.travis.yml

@@ -2,10 +2,10 @@ sudo: false
 language: python
 matrix:
   include:
+  - python: 3.9
+    env: TOX_ENV=flake8
   - python: 3.6
     env: TOX_ENV=py36
-  - python: 3.6
-    env: TOX_ENV=flake8
   - python: 3.7
     env: TOX_ENV=py37
     sudo: required
@@ -14,6 +14,9 @@ matrix:
     env: TOX_ENV=py38
     sudo: required
     dist: xenial
+  - python: "3.10"
+    env: TOX_ENV=py310
+
 install:
 - pip install tox
 script: