Lazy load model versions and enforce strict typing

[bact]

This PR reimplements PR #31 (strict typing) and incorporates the suggestion to avoid loading all versions of the SPDX model upon import spdx_python_module. This saves memory and reduces load time (see #31 (comment)).

• Pass mypy --strict
• Versions are now lazy loaded
• BREAKING CHANGE: from spdx_python_model import * now no longer load models

Existing code like this (from README) will still work:

# 1
import spdx_python_model

p = spdx_python_model.v3_0_1.Person()

This code will still work too:

# 2
from spdx_python_model import v3_0_1

p = v3_0_1.Person()

The only breaking change is this code:

# 3
from spdx_python_model import *

p = v3_0_1.Person()

The above code (no. 3) will no longer work because models (v3_0_1 etc) are no longer part of __all__.

[benjarobin]

LGTM

[bact]

I just added a __version__ export which is a common dunder name to expose a version number.

https://peps.python.org/pep-0008/#module-level-dunder-names