tox: add --diff to isort (#118)

Sometimes it complains and it is difficult to understand what exactly it wants.
The --diff option allows to know what is the format it expects.

Author: mauriciovasquezbernal

tox.ini

@@ -59,7 +59,7 @@ commands =
   black --check --diff opentelemetry-api/src/opentelemetry opentelemetry-api/tests/ opentelemetry-sdk/src/opentelemetry opentelemetry-sdk/tests/ ext/opentelemetry-ext-http-requests/src/ ext/opentelemetry-ext-http-requests/tests/ ext/opentelemetry-ext-wsgi/tests/
   pylint opentelemetry-api/src/opentelemetry opentelemetry-api/tests/ opentelemetry-sdk/src/opentelemetry opentelemetry-sdk/tests/ ext/opentelemetry-ext-http-requests/src/ ext/opentelemetry-ext-http-requests/tests/ ext/opentelemetry-ext-wsgi/tests/
   flake8 opentelemetry-api/src/ opentelemetry-api/tests/ opentelemetry-sdk/src/ opentelemetry-sdk/tests/ ext/opentelemetry-ext-wsgi/src/ ext/opentelemetry-ext-wsgi/tests/ ext/opentelemetry-ext-http-requests/src/
-  isort --check-only --recursive opentelemetry-api/src opentelemetry-sdk/src ext/opentelemetry-ext-wsgi/src ext/opentelemetry-ext-wsgi/tests ext/opentelemetry-ext-http-requests/src/
+  isort --check-only --diff --recursive opentelemetry-api/src opentelemetry-sdk/src ext/opentelemetry-ext-wsgi/src ext/opentelemetry-ext-wsgi/tests ext/opentelemetry-ext-http-requests/src/
 
 [testenv:docs]
 deps =