Add support for beancount v3

[LalitMaganti]

Switch to using beangulp which supports both v2 and v3.

Fixes: #253

[Nulifier]

This doesn't appear to work for the generic_importer_source.

I get the following error:

TypeError: expected str, bytes or os.PathLike object, not _FileMemo

This is related to the switch from _FileMemo to straight filepath strings. I managed to get past that by checking if the filepath has a name property and using that. When I got past that, I ran into the following issue:

TypeError: Importer.extract() got an unexpected keyword argument 'existing_entries' which is related to the change in call signature from the Importer.

[LalitMaganti]

Thanks for letting me know. 0c37757 should fix this.