ci: update python version

Author: guilhermearaujo

.github/workflows/main.yaml

@@ -14,10 +14,10 @@ jobs:
     steps:
     - uses: actions/checkout@v1
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Cache dependencies
       uses: actions/cache@v1