Pin RDFlib library, release 1.5.2

[mpsonntag]

This PR

• pins the RDFlib library version and the test dependent owlrl library version. This is required since major changes were introduced to RDFlib in the 5.0.0->6.0.0 release. Until these breaking changes have been fixed in code, the libraries are pinned to ensure that a pip install of odml does not lead to broken code in the rdf specific parts of the odml library.
• adds a py2.7 and py3.5 deprecation warning to the setup.py.
• prepares the library for release 1.5.2: updates in info.json, CHANGELOG and LICENSE.
• adds Python 3.9 to appveyor
• deactivates two tests for appveyor builds only: The tests test_handle_include and test_handle_repository from the TestVersionConverter class consistently fail with an SSL certification failure on the appveyor Windows builds. Upgrading the certifi package does not resolve this issue. Since these tests run fine on Linux, Mac and on the GH actions Windows builds, these tests are deactivated for Appveyor builds.

[coveralls]

Coverage decreased (-0.02%) to 81.134% when pulling 4c22a8f6413dcfa146c735473a6fb540de4499b5 on mpsonntag:rdfFixes into 174bc85dfe7b61cf59ac57e4fadcbf670b5b6e61 on G-Node:master.