Reduce Travis CI testing (#479)

.travis.yml

@@ -1,31 +1,11 @@
 language: python
-cache:
-  pip: true
-  directories:
-    - $HOME/.cache/pre-commit
-
-matrix:
-  fast_finish: true
-  include:
-    - python: 3.8
-      env: TOXENV=docs
-    - python: 3.8
-      env: TOXENV=lint
-    - python: 3.9
-    - python: 3.8
-    - python: 3.7
-    - python: 3.6
+cache: pip
+python: 3.7
 
 install: travis_retry pip install tox-travis
 
 script: tox
 
-after_success:
-  - |
-    if [[ "$TOXENV" != "docs" && "$TOXENV" != "lint" ]]; then
-      bash <(curl -s https://codecov.io/bash)
-    fi
-
 deploy:
   provider: pypi
   user: jazzband