broken IOTable backend

[mfastudillo]

I've created a fresh virtual environment that installed bw2data version 4.0 and the importer I created for the IO Tables broke. I suspect it broke the IOTable backend in general. Running the write_database() method inspired in the Exiobase3MonetaryImporter returns a type error:

TypeError: IOTableBackend.write() got an unexpected keyword argument 'signal'

I tried with the previous version 4.0.dev60 and it works