Update test environment

- Use Ubuntu 20.04, instead of 16.04 so we get more recent Python
  versions
- Use `python3 -m pip` to always address the pip that matches your
  `python3` distribution.
- Update `setuptools` and `importlib-metadata` to make sure we are
  compatible with recent versions.

.travis.yml

@@ -6,18 +6,17 @@ python:
 - '3.9'
 - '3.10'
 
-dist: bionic
+dist: focal
 os: linux
 
 jobs:
   fast_finish: true
 
 before_install:
-- python3 -m pip install -U setuptools importlib-metadata
 - python3 -m pip install coverage coveralls tox-travis
 
 install:
-- pip install -e .
+- python3 -m pip install -e .
 
 script:
 - tox