build(python): drop support for Python 3.6 and 3.7 (#267)

BREAKING CHANGE: drop support for Python 3.6 and 3.7

Closes reanahub/reana#784

setup.py

@@ -93,14 +93,14 @@
         "reana_workflow_engine_yadage",
     ],
     zip_safe=False,
-    python_requires=">=3.8",
     install_requires=install_requires,
     entry_points={
         "console_scripts": [
             "run-yadage-workflow="
             "reana_workflow_engine_yadage.cli:run_yadage_workflow",
         ]
     },
+    python_requires=">=3.8",
     extras_require=extras_require,
     setup_requires=setup_requires,
     tests_require=tests_require,