Fix GitHub CI Test

Author: wannaphong

.github/workflows/pythainlp-test.yml

@@ -23,6 +23,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip pytest wheel flake8
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        pip install tensorflow==2.1.0
         pip install torch==1.5.0 torchvision==0.6.0 -f https://download.pytorch.org/whl/torch_stable.html
         pip install .[full]
         pip install deepcut coverage coveralls