Set up a test suite

- use pytest as the test suite and
  measure test coverage with coverage.py
- add package for the test suite under tests/
- add nox session "test" to run the test suite
  for all supported Python versions
- use flake8 to lint pytest for consistent style

tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for the `lalib` library."""