Fix SQL warning in ExampleFile.get_by_path

Warning from tests:

test/selenium/test_module_provider.py::TestModuleProvider::test_readme_tab[/modules/moduledetails/fullypopulated/testprovider/1.5.0/example/examples/test-example-Example 1 README]
  terrareg/models.py:1770: SAWarning: SELECT statement has a cartesian product between FROM element(s) "example_file" and FROM element "module_version".  Apply join condition(s) between each element to resolve.
    row = conn.execute(select).fetchone()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html