fix(ci): manually remove the .tox/doc directory

closes #206

.gitlab-ci.yml

@@ -211,7 +211,11 @@ pages:
   before_script:
     - pip install tox
   script:
-    - tox -re doc
+    # remove previous artifacts from build_doc if any (cf. #206). The .tox/doc
+    # directory does not « look as a virtualenv », and tox refuses to remove it.
+    # Let's do it manually.
+    - rm -rf .tox/doc/
+    - tox -e doc
     - mv .tox/doc/tmp/html public
   artifacts:
     paths: