Add a GitHub Action to run tox

[cclauss]

Test results: https://github.com/cclauss/drf-haystack/actions

docs

FIXED. Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

FIXED. Warning, treated as error:
Failed to read intersphinx_mapping[http://docs.python.org/], ignored: SphinxWarning('The pre-Sphinx 1.0 'intersphinx_mapping' format is deprecated and will be removed in Sphinx 8. Update to the current format as described in the documentation. Hint: "intersphinx_mapping = {'': ('[http://docs.python.org/](http://docs.python.org//)', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping')

Warning, treated as error:
autodoc: failed to import module 'filters' from module 'drf_haystack'; the following exception was raised:
No module named 'pkg_resources'

py3.8-django3.2-es7.x

django.core.exceptions.ImproperlyConfigured: The default alias 'default' must be included in the HAYSTACK_CONNECTIONS setting.