CI: test build dist

.ci/build_dist.sh

+#!/usr/bin/env bash
+pip install wheel setuptools twine
+python setup.py sdist
+python setup.py bdist_wheel
+twine check dist/*
+
+# do a test install from both source and wheel
+sdist=`find dist -maxdepth 1 -name *.tar.gz`
+wheel=`find dist -maxdepth 1 -name *.whl`
+
+for target in $sdist $wheel; do
+    python -m venv test.venv
+    . test.venv/bin/activate
+    pip install --upgrade pip setuptools
+    pip install $target
+    deactivate
+    rm -r test.venv
+done

.gitlab-ci.yml

 stages:
  - test
+ - build
 
 test:3.7:
   stage: test
@@ -30,3 +31,13 @@ test:3.9:
 
   script:
     - bash ./.ci/unit_tests.sh
+
+build:dist:
+ stage: build
+ image: python:3.8
+
+  tags:
+    - docker
+
+  script:
+    - bash ./.ci/build_dist.sh