CI: Bump python version and fix pip install (#6)

    See pypa/setuptools#3293

Author: s3bw

.travis.yml

@@ -1,11 +1,12 @@
 dist: xenial
-cache: pip
 language: python
 python:
   - "3.6"
   - "3.7"
   - "3.8"
+  - "3.9"
 install:
   - pip install -r requirements-dev.txt
+  - pip install --upgrade 'importlib-metadata > 4'
   - pip install .
 script: pytest tests/