Automated tests with pytest

[soerenetler]

I think the examples folder should be rewritten as tests. This would still serve their purpose for examples but also give test to find potential bugs earlier. I already created a first version what this could look like. The TOKEN, PHONE_NUMBER_ID and RECIPIENT_ID could be put in the github secrets and the tests could be execute after commit on the main branch. For local testing everyone has to provide his/her own environment secrets.

[soerenetler]

This is implemented in #38