Start testing on Python 3.7

and stop testing on Python 3.6
because 3.7 is the minimum required by matplotlib 3.4

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.6", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
       - uses: actions/checkout@v2