ENH: add votable.parquet as format alias for unified I/O

[bsipocz]

I think it would be beneficial to enable the sub-formats accepted by write methods to be available for read as well, even when the format is automatically recognized as a top-level one.

The one known example is votable.parquet, but there may be others, too, where reading format='votable works, but votable.parquet doesn't.

>>> from astropy.table import Table
>>> Table.read('astropy/io/votable/tests/data/parquet_binary.xml', format='votable')
>>> Table.read('astropy/io/votable/tests/data/parquet_binary.xml', format='votable.parquet')

[pllim]

(oops comment on the issue instead of PR, sorry for the noise here.)