Update gitlab-ci.yml

.gitlab-ci.yml

-test:
-    script:
-        - cp /keys/.pypirc ~/.pypirc
-        - virtualenv env
-        - source env/bin/activate
-        - XDG_CACHE_HOME=/cache pip3 install -U pip wheel setuptools pytest pytest-django coverage
-        - XDG_CACHE_HOME=/cache pip3 install -U "git+https://github.com/pretix/pretix.git@master#egg=pretix&subdirectory=src"
-        - python setup.py develop
-        - make
-        - coverage run -m py.test tests
-        - coverage report
-    tags:
-        - python3
-style:
+---
+stages:
+  - linting
+# Change pip's cache directory to be inside the project directory since we can
+# only cache local items.
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+cache:
+  paths:
+    - .cache
+
+docformatter:
+  stage: linting
+  image: python:3
+  before_script:
+    - pip3 install -U pip docformatter
   script:
-        - cp /keys/.pypirc ~/.pypirc
-        - virtualenv env
-        - source env/bin/activate
-        - XDG_CACHE_HOME=/cache pip3 install -U pip wheel setuptools isort docformatter black flake8 check-manifest
-        - XDG_CACHE_HOME=/cache pip3 install -U "git+https://github.com/pretix/pretix.git@master#egg=pretix&subdirectory=src"
-        - python setup.py develop
     - docformatter --check -r .
+
+black:
+  stage: linting
+  image: python:3
+  before_script:
+    - pip3 install -U pip black
+  script:
     - black --check .
+
+isort:
+  stage: linting
+  image: python:3
+  before_script:
+    - pip3 install -U pip isort
+  script:
     - isort -c .
-        - flake8 .
-        - check-manifest .
-    tags:
-        - python3
-pypi:
+
+flake8:
+  stage: linting
+  image: python:3
+  before_script:
+    - pip3 install -U pip flake8
   script:
-        - cp /keys/.pypirc ~/.pypirc
-        - virtualenv env
-        - source env/bin/activate
-        - XDG_CACHE_HOME=/cache pip3 install -U pip wheel setuptools twine check-manifest
-        - XDG_CACHE_HOME=/cache pip3 install -U pretix
-        - python setup.py develop
-        - python setup.py sdist bdist_wheel
-        - check-manifest .
-        - twine check dist/*
-        - twine upload dist/*
-    tags:
-        - python3
-    only:
-        - pypi
-    artifacts:
-        paths:
-            - dist/
+    - flake8 .