setup: Drop support for EOL python < 3.8

The new PySNMP package (pysnmplib) is not available for Python < 3.8. At
the same time, we only want to support the actively maintained
(supported) Python versions, which at the time of writing are 3.8 to
3.12. Hence, update the Python requirement on setup.py and drop the CI
jobs testing Python < 3.8.

.gitlab-ci.yml

@@ -57,14 +57,6 @@ stages:
     # Exercise all imports triggered when launching the CLI
     - blocksat-cli --version
 
-unit-tests-py36:
-  extends: .test
-  image: python:3.6
-
-unit-tests-py37:
-  extends: .test
-  image: python:3.7
-
 unit-tests-py38:
   extends: .test
   image: python:3.8

setup.py

@@ -48,4 +48,4 @@
         "Programming Language :: Python :: 3 :: Only",
         "License :: OSI Approved :: GNU General Public License v3 (GPLv3)"
     ],
-    python_requires='>=3')
+    python_requires='>=3.8')