Feature/deploy

[alshapton]

This PR addresses Issue #15 .

It:

• Checks the code using SonarCloud
• Uses flake8 to check for Python syntax errors
• Uses flake8 to scan the code (in addition to SonarCloud)
• Runs the built-in tests with Pytest
• Runs accessibility checks with pa11y
• Publishes to PyPI (if on a release branch)
• Publishes rudimentary documentation to READTHEDOCS.org (if on a release branch)

This build pipeline runs on a matrix of MacOS/Ubuntu with Python versions: 3.7, 3.8, 3.9, 3.10 and pypy-3.8

It also introduces the GitFlow branching strategy (to be fully implemented within the main repository by @mrwilson, however the travis build is deprecated and will be removed in a future PR)

There is a new CONTRIBUTING.md file
Restructured directories slightly