add Python 3.8

Author: akhmerov

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7]
+        python-version: [3.5, 3.6, 3.7, 3.8]
 
     steps:
     - uses: actions/checkout@v2