misc/py-spdx-tools: Update to 0.6.1

• Enable build for all Python versions because Python 3 is supported since the 0.5.x release. This became more evident via tests by using pytest.

• Make the port concurrent safe as it installs a script and some (optional) example files.

• Also refine the "do-test" target by using devel/py-pytest which gives a better overview of the test results.

Changelog since 0.5.4:

https://github.com/spdx/tools-python/compare/v0.5.4...eac4e90