Motivation

I've been using the Py2 SCL library in the stepcode library to good effect for some IFC experiments. I now would like to integrate this with adjacent libraries that have a Py3 dependency. Considering Python2 is sunset, and there's interest (Issue 407) from the stepcode community in a Py3 port, it seemed like a good time to start. This PR is the result.

Summary of Changes

Changes fell in two categories:

1. Those changes critical to make SCL run under Py3 at all
2. Convenience features like egg packaging that will hopefully make working with SCL easier in the future.

Changes on (1) were minor (updating the use of the new print function, migrating all modules to use new-style classes).

Changes on (2) were more significant. They include:

1. Creating a proper setup.cfg configuration to allow us to build SCL as a Python Package and automatically install required dependencies. This also opens the door to publishing the library on PyPI, so users could install with pip and friends.
2. Ensure the test suite can run using python3 -m unittest discover
3. Adding a new set of test cases, test_parser.py, that do a basic sanity check on the parser logic against a sample Part 21 file.
4. Adding a separate LICENSE file (that mirrors those already shown in line at the top of SCL files), and basic README that provides a starting point for newcomers.

The only "breaking change" is a test case file, test_unitary_schemas.py, that I prefixed with an underscore to avoid being called by unittest discover. test_untiary_schemas.py relies on Python modules autogenerated by exp2python in the parent directory. Those autogenerated modules make use of a decorator, called @apply, I haven't been able to find/grep in the source tree. The tests won't be able to run until this decorator definition is found/written and put back in the sys.path. For now I've added this note in the test case file with an open invitation for others to bring it back to life if they know where the missing dependencies are. If the meaning or source for @apply can be found, it doesn't seem like we're far off from resurrecting these particular tests.

Validation

I'm able to successfully build the package (python3 -m build) and run all unit tests (python -m unittest discover - 101/101 pass) on my local machine. I can also build the full stepcode project using the usual cmake workflow from scratch without errors.