Resolve build problems in 3.3.1.0

[cderici]

Description

This solves 2 problems with the latest release (also described in this comment):

1. We depend on the packaging module, but didn't include it in the setup.py (because our testing and dev environments have tox installed and tox satisfies that requirement for itself, so we didn't notice).
2. We accidentally depend on the static VERSION file where we keep track of the version of pylibjuju.

This change fixes #1025, solving these issues by:

1. Adding the packaging to the required modules.
2. Removing the file VERSION, and moving the static version information into the version.py module (just like in Juju) and use that information from there. The places that dynamically use that information (outside of the juju modules) are i) setup.py to get the version info into the build, ii) Makefile to use it in the release target iii) docs/config.py for building the docs. All of these spots are updated to use the info within the version.py.

QA Steps

We'll use the steps described in #1025. After pulling the branch in this PR;

• Create a new virtual env:

python -m venv venv

• Activate it:

source venv/bin/activate

• Build pylibjuju at this spot:

python setup.py sdist

• The sdist build will create a dist/juju-3.3.1.0.tar.gz, so install that:

pip install dist/juju-3.3.1.0.tar.gz

• Now try to import a Controller.

python3 -c "from juju.controller import Controller"

Without the (1)st change in this PR, this command would result in:

ModuleNotFoundError: No module named 'packaging'

And without the (2)nd change, this command would error with

FileNotFoundError: [Errno 2] No such file or directory: '......./venv/lib/python3.10/site-packages/VERSION'

But with this change, we shouldn't see either (or any other) error. I.e., the command should succeed with no problems.

Notes & Discussion

Note that this changes the release process. In particular where we need to manually change the version information is moved into the version.py module (the release process document will be updated right before this PR is merged).

Moreover, we need to make a patch release (3.3.1.1) after this for these changes to take effect (i.e. for pylibjuju to be usable in the environments where it's installed as a dependency).

JUJU-5484

[jack-w-shaw]

Imports should be at the top no?

[cderici]

This one should stay there because it won't work before the sys.path.insert(0, os.path.abspath('..')) line above

[jack-w-shaw]

Do we still need to strip?

[cderici]

We currently don't need to, but the line that's coming from will be manually edited by people who're making releases (replaces the file VERSION) in the future, so I thought this would cleanup any silly mistakes like "4.2.0 " (whitespace in the end).

[cderici]

/merge