[python] Group and array-level meta data written as bytes should be preserved as bytes

[mojaveazure]

When writing group- or array-level meta data, bytes are cast to TILEDB_STRING_UTF8 instead of TILEDB_STRING_ASCII, resulting in the meta data being read back as str instead of bytes

>>> # Write metadata as both `str` and `bytes`
>>> grp = tiledbsoma.Collection.create("temp")
>>> grp.metadata["str"] = "test_string"
>>> grp.metadata["bytes"] = b"test_bytes"
>>> grp.close()

>>> # Read it back
>>> grp = tiledbsoma.Collection.open("temp")
>>> {x: type(grp.metadata[x]) for x in ("str", "bytes")}
{'str': <class 'str'>, 'bytes': <class 'str'>}