added empty __init__.py file so that coverage is correctly computed

tests/__init__.py

@@ -0,0 +1,24 @@
+# -*- coding: UTF-8 -*-
+
+"""This file seems to be necessary for `pytest-cov` to correctly compute code
+coverage, when the tests are run from the project root directory with the
+command:
+
+  $ pytest
+
+Though this file does not seem to be necessary when running the tests with the
+command:
+
+  $ python -m pytest
+
+This seems to be a known issue with the `pytest` and `coverage` packages:
+
+  * https://github.com/pytest-dev/pytest-cov/issues/401
+  * https://stackoverflow.com/q/47287721/13026442
+
+There's probably a clever way to fix this using `pytest.ini` or `.coveragerc`
+files, but until I figure that out, this will have to do.
+
+"""
+
+#+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++#