Fix #60, do not package tests directory

According to the following link we can remove the call to
`find_packages` because the default behavior will do the correct thing:

https://setuptools.pypa.io/en/latest/userguide/package_discovery.html#flat-layout

setup.py

@@ -1,10 +1,7 @@
-from setuptools import (
-    find_packages,
-    setup,
-)
-
 import os
 
+from setuptools import setup
+
 import dlint
 
 requirements_filename = os.path.join(
@@ -32,7 +29,6 @@
     long_description=long_description,
     long_description_content_type='text/markdown',
     url=dlint.__url__,
-    packages=find_packages(),
     license=dlint.__license__,
     classifiers=[
         'Environment :: Console',