Fix unclosed file warning

No functional changes - just silencing a ResourceWarning by ensuring the file handle is explicitly closed. ResourceWarning is typically ignored by default but some distributions enable it.

$ python -m pytest -Wdefault tests
...
mock/tests/test_config_loader.py::TestConfigLoader::test_config_paths
  /home/me/mock/mock/py/mockbuild/config.py:722: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/me/mock/mock/tests/data/config-001/site-defaults.cfg' mode='r' encoding='UTF-8'>
    content = open(config_file).read()