v0.5.2 release on PyPi

• bump up versions of dependencies to fit Py 3.7+
• improve testing based on pytest; kudos @Mec-iS
• allow config for rdflib.Store plugins, e.g., the excellent Oxrdflib; kudos @paoespinozarias

What's Changed

• updated docs by @ceteri in #234
• add DH badge by @ceteri in #235
• add DH badge by @ceteri in #236
• Implement tests modules by @Mec-iS in #238
• Storage plugin by @ceteri in #239
• allow configurable rdflib.Store plugins, e.g., Oxrdflib by @ceteri in #240

Full Changelog: v0.5.1...v0.5.2