GC-45: Update copyright information:

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

GC-45: Upgrade to cryptographyt version 42.0:

- Passing in backend is no longer required/possible.

GC-37: Use KeyGenerator instead of generate_private_key in unit tests:

- Replaced all uses of generate_private_key with KeyGenerator
invocations that explicitly request 2048-bit RSA.

GC-37: Refactor key specification handling:

- Perform the key specification parsing within CLI module itself,
don't do it via crypto module.
- Pass-in tuple consisting out of algorithm and associated parameters
into the init command instead of key generator.
- Updated all tests to accomodate the change in init function
signature.
- Simplify the KeyGenerator class.
- Do not test if KeyGenerator class sets the properties via
constructor - it is sufficient to test string represenation and key
generation.

GC-37: Added support for requesting custom RSA key size when initialising the CA hierarchy:

- Added functional test.
- Added unit tests.
- Added new CLI option for specifying the algorithm.
- Implemented KeyGenerator factory-like class that can be called to
generate a private key with desired specification.
- The init init function now accepts a callable that is used to
generate private keys.
- The generate_ca_hierarchy function now accepts a callable that is
used to generate private keys.
- Updated existing unit tests to cope with changes to the init
and generate_ca_hierarchy function signatures.
- Updated existing unit tests to cope with changes to existing
functionality.
- Updated existing functional tests to cope with changes in command
output.

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 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.

GC-20: Implemented status command for initialised directory:

- Added functional test.
- Implemented showing CA hierarchy, server and client certificate
information for initialised directories.
- Implemented helper functions for generating string representation of
DNs, DNS subject alternative names, and date range.
- Return correct error when calling status command on uninitialised
directory.
- Updated gitignore to exclude *.pyc from repository.
- Implemented unit tests.