Add missing `__init__.py` to prevent import errors

Without the `tests/__init__.py` file, the tests directory was not
considered a package and therefore, running:

    $ pytest

would fail with import error notifications. This fixes that.

1 files changed, 0 insertions, 0 deletions

diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tests/__init__.py