xlrd / xlwt requirements not specified

[ketaro]

When attempting to export a form on a fresh install, get the following error:

  File "/home/nicka/repos/xxxxx/env/lib/python3.8/site-packages/aldryn_forms/admin/views.py", line 96, in done
    response = HttpResponse(dataset.xls, content_type=content_type)
AttributeError: 'Dataset' object has no attribute 'xls'

It's easily fixed by doing a pip install "tablib[xls]", but the dependancies for xlrd and xlwt should be added.