Drop Python 3.6 support, add 3.9 support

3.10 and 3.11 are blocked by
vinayak-mehta/pdftopng#17 (comment)

.github/workflows/tests.yml

@@ -4,10 +4,10 @@ on: [pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: ["3.7", "3.8"]
 
     steps:
       - uses: actions/checkout@v3
@@ -28,7 +28,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.9]
+        python-version: ["3.9"]
 
     steps:
       - uses: actions/checkout@v3

setup.py

@@ -73,9 +73,9 @@ def setup_package():
             # Trove classifiers
             # Full list: https://pypi.python.org/pypi?%3Aaction=list_classifiers
             "License :: OSI Approved :: MIT License",
-            "Programming Language :: Python :: 3.6",
             "Programming Language :: Python :: 3.7",
             "Programming Language :: Python :: 3.8",
+            "Programming Language :: Python :: 3.9",
         ],
     )