Fix pypi upload in CI

Twine has been upgraded to 3.0.0 and now depends on gcc. So in order to
deploy ecommerce, we can either install gcc or downgrade twine to 2.0.0.
We chose the latter, as pinning dependencies is a good practice. In the
future, we'll probably have to upgrade twine and install gcc.

.gitlab-ci.yml

@@ -30,7 +30,7 @@ deploy:image:
 deploy:pypi:
   script:
     - apk add --no-cache make
-    - pip3 install -U setuptools twine
+    - pip3 install setuptools==41.6.0 twine==2.0.0
     - python3 setup.py sdist
     - twine upload --skip-existing dist/tutor-discovery*.tar.gz
   only: