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-28: Increase timeout for running interactive commands in tests:

- Running the functional tests from within Vagrant machine that
involve interactive commands could result in timeout due to slower
execution. Double the timeout to avoid such issues. Long-term might
be worth it to look into why this happens at all.

Noticket: Compatibility fixes for Python 3.4:

- Fixed invocation of pexepect.spawnu to convert the passed-in
arguments explicitly into a list. Necessary since Python 3.4 can't
use the *args construct outside of assignment.
- Updated the certificate_to_pem function to return str instead of
bytes. Necessary since Python 3.4 does not support things like b"%s"
% mybytes.
- Fixed test for existence of help CLI command. Previous code was
referencing a wrong/non-existent function help (the actual name has
to be help_ in order not to shadow the built-in function).
- Updated unit test invocations that use the read_certificate
function.
- Updated tests for the read_certificate function.

GC-22: Updated renew command to allow reading of CSR from stdin:

- Implemented functional test covering reading of CSR from stdin for
the renew command.
- Updated renew command CLI help.
- Updated renew command to read CSR from stdin if passed-in path is
set to '-'.
- Implemented relevant unit tests.

GC-22: Updated server command to allow reading of CSR from stdin:

- Implemented an additional helper for functional tests for running
interactive commands.
- Implemented functional test covering passing-in CSR to the server
command via stdin (interactively).
- Updated server command implementation.
- Implemented utility function for reading input from user.
- Implemented utility function for reading CSR from string in (in
OpenSSL-style PEM format)
- Fixed some missing imports in the custom pytest fixture.
- Implemented relevant unit tests.