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/async/.travis.yml

@@ -0,0 +1,17 @@
+# ref: https://docs.travis-ci.com/user/languages/python
+language: python
+python:
+  - "3.10"
+  - "3.11"
+  - "3.12"
+  - "3.13"
+  - "3.14"
+  # uncomment the following if needed
+  #- "3.14-dev"  # 3.14 development branch
+  #- "nightly"  # nightly build
+# command to install dependencies
+install:
+  - "pip install -r requirements.txt"
+  - "pip install -r test-requirements.txt"
+# command to run tests
+script: pytest --cov=easybill_generated_async