Fix setup 'packages' argument

So that we don't install a 'tests' module globally in each env

setup.py

@@ -30,7 +30,7 @@ setup(
     author="3YOURMIND GmbH",
     author_email="david.wobrock@gmail.com",
     license="Apache License 2.0",
-    packages=find_packages(exclude=["tests/"]),
+    packages=find_packages(include=["django_migration_linter*"]),
     install_requires=[
         "django>=1.11",
         "appdirs>=1.4.3",