Include pretix to CI installations

.gitlab-ci.yml

@@ -13,7 +13,7 @@ black:
   stage: linting
   image: python:3
   before_script:
-    - pip3 install -U pip black
+    - pip3 install -U pip pretix black
   script:
     - black --check .
 
@@ -21,7 +21,7 @@ isort:
   stage: linting
   image: python:3
   before_script:
-    - pip3 install -U pip isort
+    - pip3 install -U pip pretix isort
   script:
     - isort -c .
 
@@ -29,7 +29,7 @@ flake8:
   stage: linting
   image: python:3
   before_script:
-    - pip3 install -U pip flake8
+    - pip3 install -U pip pretix flake8
   script:
     - flake8 .
 
@@ -37,6 +37,6 @@ check-manifest:
   stage: linting
   image: python:3
   before_script:
-    - pip3 install -U pip check-manifest
+    - pip3 install -U pip pretix check-manifest
   script:
     - check-manifest .