Update Travis build to use multiple Python versions 🛠️

.travis.yml

@@ -2,23 +2,22 @@ language: python
 
 python:
   - 3.6
+  - 3.7
+  - 3.8
 
 install:
   - "pip install -U pip"
   - "pip install poetry"
-  - "poetry install --no-interaction"
+  - "poetry install -E tf --no-interaction --no-ansi"
 
 jobs:
   include:
     - stage: codestyle
-      python: 3.6
       script:
         - "make lint"
     - stage: test
-      python: 3.6
       script:
         - "make test"
     - stage: coverage
-      python: 3.6
       script:
         - "make coverage"

maru/resource/rnn/__init__.py

@@ -26,10 +26,10 @@ def load_tags() -> Dict[int, Tag]:
 def load_tagger() -> 'tensorflow.keras.Model':
     try:
         import tensorflow.keras
-    except ModuleNotFoundError:
+    except ModuleNotFoundError as e:
         raise ImportError(
             'RNN tagger requires TensorFlow. You can install it with "pip install maru[tf]"'
-        )
+        ) from e
     # this restrains tensorflow from allocating all of available GPU memory
     config = tensorflow.compat.v1.ConfigProto()
     config.gpu_options.allow_growth = True