Remove ubuntu 20.04 from CI, losses test coverage for Python 3.6 an 3.7 (#161)

Author: EnricoMi

.github/workflows/ci.yml

@@ -30,8 +30,6 @@ jobs:
       # EOL Python versions
       include: >
         [
-          {"os": "ubuntu-20.04", "python-version": "3.6"},
-          {"os": "ubuntu-20.04", "python-version": "3.7"},
           {"os": "ubuntu-22.04", "python-version": "3.8"},
         ]
 
@@ -132,7 +130,5 @@ jobs:
       # EOL Python versions
       include: >
         [
-          {"os": "ubuntu-20.04", "python-version": "3.6"},
-          {"os": "ubuntu-20.04", "python-version": "3.7"},
           {"os": "ubuntu-22.04", "python-version": "3.8"},
         ]