Allow for byte returns from pg version call

[lossyrob]

I ran into a case where the check on this line: https://github.com/stac-utils/pgstac/blob/main/pypgstac/pypgstac/db.py#L234 failed due to the row value being bytes, not str. Forcing a .decode() on the value if it's type(bytes) fixed the issue. Not sure why this would return str some of the time and bytes others, or if this also could be a problem for the version method on the same class.