Add unit tests for authentication and webhook parsing

- Implement tests for basic and bearer authentication headers in `test_auth.py`.
- Create tests for the `EasybillWebhookParser` in `test_webhooks.py`, covering JSON and form-encoded payloads, as well as a generic parse and acknowledgement method.

generated/sync/tox.ini

@@ -0,0 +1,9 @@
+[tox]
+envlist = py3
+
+[testenv]
+deps=-r{toxinidir}/requirements.txt
+     -r{toxinidir}/test-requirements.txt
+
+commands=
+   pytest --cov=easybill_generated_sync