Revert "add python 3.9 to tox matrix"

This reverts commit 02c0f11.

Author: vgvoleg

.github/workflows/tests.yaml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.8, 3.9]
+        python-version: [3.8]
         environment: [py, py-tls, py-proto3, py-tls-proto3]
         folder: [ydb, tests]
         exclude: