Createsend

* Updated cacert.pem. From [92](https://github.com/campaignmonitor/createsend-python/pull/92)
* Updates various URLs to use HTTPS
* Fixes some URLs in CONTRIBUTING.md
* Use GH Workflow badge instead of inactive Travis one in documentation. From [91](https://github.com/campaignmonitor/createsend-python/pull/91)

* This release updates the add() and update() methods in lib/subscriber.py, enabling users to add subscribers' mobile numbers along with their consent to receive SMS messages.
* Updated subscriber.py within samples to showcase new use cases resulting from these updates. [PR](https://github.com/campaignmonitor/createsend-python/pull/90)

* Improved error handling for 400 and 500 errors: Now displays error codes and error messages for better debugging.

* Fix deprecation warnings from [86](https://github.com/campaignmonitor/createsend-python/pull/86)

* Tidy up of code from [83](https://github.com/campaignmonitor/createsend-python/pull/83)
* Fixing tox config
* Github workflow added
* Breaking: Python versions 3.7 and prior will no longer work with this version of the wrapper. The actual wrapper code might work with Python 3.6 and 3.7. However, some of the tox testing around it may not work with those versions. So we've dropped support for that.

* Tidy up of code from [84](https://github.com/campaignmonitor/createsend-python/pull/84)