GC-44: Added support for Python 3.12:

- Updated documentation.
- Updated release notes.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-43: Fix provisioning script for Vagrant machine:

- Version patch number can be longer than one digit, make sure to
strip it correctly.

GC-43: Added support for Python 3.11:

- Updated documentation.
- Updated release notes.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-42: Added support for Python 3.10:

- Updated documentation.
- Updated release notes.
- Updated functional tests (small tweak for help output change).
- Added signing key for the Python 3.10 and 3.11 releases.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

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-39: Added support for Python 3.9:

- Updated release notes
- Updated functional test for changes in help output in Python 3.9.
- Updated comments on public signing keys used for validating the
Python archives when using Vagrant provisioning.
- Updated supported Python versions in package configuration file.
- Updated Python versions used for tests.

GC-33: Added support for Python 3.8:

- Updated release notes.
- Updated documentation.
- Updated Python versions used for tests.
- Updated supported Python versions in package configuration file.

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.

GC-28: Build and install supported Python versions with custom script:

- Drops external dependency on pyenv.
- Makes the process actually simpler because pyenv does not play
nicely with some built-in tools likes virtualenv/virtualenvwrapper.

GC-28: Added initial set-up for running tests within Vagrant machine:

- Added Vagrantfile that deploys Debian 9 Stretch and provisions it.
- Added provisioning script that will set-up multiple Python versions.
- Updated development instructions to include information on how to
use Vagrant to run all the tests.
- Ignore Vagrant artifacts in gitignore.