ci: add default .gitlab-ci

.gitlab-ci.yml

+---
+default:
+  image: python:3.7
+
+include:
+  - ".gitlab-ci-extended.yml"
+  - project: "open-source/gitlab-ci-templates"
+    ref: "branch/default"
+    file:
+      - "templates/no-duplicated-ci-pipelines.yml" # use workflow to avoid duplicated pipelines
+      - "templates/lint/flake8.yml" # will do the equivalent of 'tox -e flake8'
+      - "templates/lint/check-manifest.yml" # will do the equivalent of 'tox -e check-manifest'
+      - "templates/tests/py3.yml" # will do the equivalent of 'tox -e py3'
+      - "templates/create-release-on-heptapod.yml" # this will create a release on heptapod
+      - "templates/upload-to-pypi.yml" # on a new mercurial tag (expected to be done with release-new), will push a release on pypi
+
+stages:
+  - lint
+  - tests
+  - release
+  - publish