GC-45: Update copyright information:

- Changes to code base were previously done in 2020 as well (based on
git history).

GC-45: Drop support for Python 3.5, 3.6, and 3.7:

- Updated documentation, removing references to versions as being
supported.
- Switched the Vagrant test machine to Debian Bullseye (still in LTS
support) to match the main development environment.
- Updated functional tests for output changes in OpenSSL CLI from
Debian Bullseye.
- Updated Tox configuration and instructions.
- Dropped workaround for Python 3.5.
- Updated package requirements.
- Updated release notes.

GC-35: Drop support for Python 3.4:

- Updated documentation, removing Python 3.4 as supported version.
- Updated documentation to include support for Python 3.7 (this was
already supported, but documentation did not list it in a couple of
places).
- Updated Tox configuration with list of supported Python
versions.
- Build Python 3.5.x for full test against all Python versions in
order to make it independent of underlying distribution Python
version.
- Update package requirements.
- Updated release notes.