Fix deprecation warning by making h5py file mode explicit

Fix the following deprecation warning:

    H5pyDeprecationWarning: The default file mode will change to 'r'
    (read-only) in h5py 3.0. To suppress this warning, pass the mode
    you need to h5py.File(), or set the global default
    h5.get_config().default_file_mode, or set the environment
    variable H5PY_DEFAULT_READONLY=1. Available modes are: 'r', 'r+',
    'w', 'w-'/'x', 'a'. See the docs for details.

ligo/em_bright.py

@@ -159,7 +159,7 @@ def source_classification_pe(posterior_samples_file, hdf5=True,
     (0.0, 0.0)
     """
     if hdf5:
-        data = h5py.File(posterior_samples_file)
+        data = h5py.File(posterior_samples_file, 'r')
         engine = list(data['lalinference'].keys())[0]
         samples = data['lalinference'][engine]['posterior_samples'][()]
         mc_det_frame = samples['mc']

ligo/tests/test_em_bright.py

@@ -11,7 +11,7 @@ from .. import em_bright
 def test_source_classification_pe():
     with NamedTemporaryFile() as f:
         filename = f.name
-        with h5py.File(f) as tmp_h5:
+        with h5py.File(f, 'w') as tmp_h5:
             data = np.array([(1.2, 1.0, 0.0, 0.0, 100.0), (2.0, 0.5, 0.99, 0.99, 150.0)],
                             dtype=[('mc', '<f8'), ('q', '<f8'),
                                    ('a1', '<f8'), ('a2', '<f8'), ('dist', '<f8')])