build(python): drop support for Python 3.6 and 3.7 (reanahub#235)

BREAKING CHANGE: drop support for Python 3.6 and 3.7

Closes reanahub/reana#784

.github/workflows/ci.yml

@@ -143,7 +143,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ['3.8', '3.9']
 
     steps:
       - name: Checkout

setup.py

@@ -75,7 +75,7 @@
     },
     zip_safe=False,
     include_package_data=True,
-    python_requires=">=3.6",
+    python_requires=">=3.8",
     install_requires=install_requires,
     extras_require=extras_require,
     setup_requires=setup_requires,
@@ -87,8 +87,6 @@
         "License :: OSI Approved :: MIT License",
         "Operating System :: OS Independent",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.6",
-        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: Implementation :: CPython",