[CHORE] revert the v3.0.0 that was triggered by error

[jkowalleck]

python-semantic-release has/had an error (besides ##285),
that triggered a release v3.0.0 when it should have triggered v2.7.1 ...

v2.7.1 was triggered manually

the remains of v3.0.0 need to be removed,
as an actual v3 will be released eventually.

• remove the v3.0,0 tag
• remove 3.0.0 from https://github.com/CycloneDX/cyclonedx-python-lib/blob/main/CHANGELOG.md
  via remove v3 from CHANGELOG #286 #287
• remove 3.0.0 from GH releases: https://github.com/CycloneDX/cyclonedx-python-lib/releases/tag/v3.0.0
• revert 3.0.0 from PyPI: https://pypi.org/project/cyclonedx-python-lib/

[jkowalleck]

@madpah FYI

[madpah]

Thanks @jkowalleck - had not realised all this other stuff had happened too!