update Python tests

- drop 2.7 (see #152)
- add 3.6
- note: 3.9 not supported in MDAnalysis 1.1.1

Author: orbeckst

.github/workflows/gh-ci.yaml

@@ -27,12 +27,12 @@ jobs:
     strategy:
         fail-fast: false
         matrix:
-          os: [ubuntu-latest, ]
-          python-version: [2.7, 3.7, 3.8]
+          os: [ubuntu-latest,]
+          python-version: [3.6, 3.7, 3.8]
           include:
             - name: macOS_bigsur_py39
               os: macOS-11
-              python-version: 3.8
+              python-version: 3.9
             - name: macOS_catalina_py37
               os: macOS-10.15
               python-version: 3.7
@@ -140,7 +140,7 @@ jobs:
       run: |
         conda install setuptools twine
 
-    - name: install PMDA
+    - name: package PMDA
       run: |
         python setup.py sdist