We were enabling explicit_package_bases to solve the duplicate conftest.py error, but then when we remove the --ignore-missing-imports option by upgrading the mypy action, it turns out that mypy can't find any of the asf_tools.* imports. When we remove explicit_package_bases and instead exclude conftest.py files, we're left with 27 errors to address. This seems reasonable, though I wonder if there's a better way that doesn't exclude the conftest.py files.

Edit: The solution is to add a tests/__init__.py file, which is also recommended by pytest.