travis: Simplify to use pytest-cov

.travis.yml

@@ -7,15 +7,9 @@ python:
   - 3.7
   - pypy3
 cache: pip
-before_install:
-  - export PIP_USE_MIRRORS=true
-  - pip install -U pip setuptools
-  - pip install -U pytest mock # https://github.com/travis-ci/travis-ci/issues/4873
-  - pip install coveralls
-  - git config --global user.name 'travis-ci'
-  - git config --global user.email '[email protected]'
 install:
-  - "pip install -e ."
-script: coverage run --source=unihan_db setup.py test
+  - pip install -e .
+  - pip install -r requirements/test.txt
+script: py.test --cov=unihan_db
 after_success:
   - bash <(curl -s https://codecov.io/bash)