chore: update template test_main.py

{{ cookiecutter.package_name }}/tests/test_main.py

 """Sample test module."""
 
-from {{ cookiecutter.package_slug }} import main
+import {{ cookiecutter.package_slug }}
 
 
 def test_main():
     """Test main function."""
-    assert main()
+    assert {{ cookiecutter.package_slug }}.main()