Fix unclosed file warning on debug python

[ngoldbaum]

Debug builds of python have a warning about this that displays every time I run numpy's spin config:

nathan at rous in ~/Documents/numpy on rename-numpy-core!
± python -m spin test
Invoking `build` prior to running tests:
/home/nathan/.pyenv/versions/3.10.9-debug/lib/python3.10/site-packages/spin/cmds/meson.py:99: ResourceWarning: unclosed file <_io.TextIOWrapper name='build/meson-info/meson-info.json' mode='r' encoding='UTF-8'>
  meson_info = json.load(open(meson_info_fn))
ResourceWarning: Enable tracemalloc to get the object allocation traceback

[stefanv]

Thanks!