Deprecate support for Python 3.6

[madpah]

Python 3.6 is reached its end of life as of December 2021.

Questions for open discussion:

1. Should cyclonedx-python-lib continue to provide support for Python 3.6?
2. If Yes to (1), for how long should we look to maintain support for Python 3.6?

FYI: @jkowalleck

[jkowalleck]

my point of view: drop support or outdated python versions.
whoever needs support for old python might not use latest version of CycloneDX-py ...

[madpah]

Given we've had no objections, we will be moving this deprecation into the forthcoming 3.0.x release.

[hakandilek]

#325 is for dropping 3.6 on dev/3.0.0 branch