Migrate to pyairtable 2.x

[gasman]

Fixes #71. Incorporates #75

[gasman]

The API changes from the airtable-python-wrapper package to pyairtable 2.x are summarised at https://pyairtable.readthedocs.io/en/stable/migrations.html#api-changes-in-1-0 - essentially rather than retrieving a client object with Airtable(base_key, table_name, api_key) there's an extra level of indirection to create an Api object and then retrieve a Table from it - after that things work mostly as before, just with some renames and search being a special case of all.

This means some fiddly changes in mock_airtable.py, but we expose the table object as _table so that we can continue fiddling with return_value as before.

[laymonage]

I code-searched the API changes and they seem to have been handled here, thanks!