Add extra metadata to setup.py

[joshuagl]

Add the license and the content type of long_description. Add extra
supported versions of Python to the Classifiers list.

Please fill in the fields below to submit a pull request. The more information
that is provided, the better.

Fixes issue #: N/A

Description of the changes being introduced by the pull request:

• Add missing license field to metadata in setup.py
• Add long_description_content_type field to metadata in setup.py
• Add additional items to the classifiers list for supported Python versions

Please verify and check that the pull request fulfils the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[coveralls]

Coverage remained the same at ?% when pulling d1e5cf3 on joshuagl:joshuagl/minor into 7c183ec on secure-systems-lab:master.

[lukpueh]

Solid PR. Many thanks, @joshuagl!