Add validation checks before attempting to publish on PyPI

* in hopes that publish failures will be less confusing from now on
* also, to detect errors (e.g. missing files in MANIFEST.in) earlier

.gitlab-ci.yml

@@ -19,7 +19,9 @@ pypi-release:
   rules:
     - if: '$CI_COMMIT_TAG =~ /^v\d+/'
   script:
-    - pip3 install --upgrade pip build twine
+    - pip3 install --upgrade pip build twine check-manifest
     - rm -f dist/*
+    - python3 -m check_manifest
     - python3 -m build
+    - python3 -m twine check dist/*
     - TWINE_USERNAME=${PYPI_USER} TWINE_PASSWORD=${PYPI_PASSWORD} python3 -m twine upload --skip-existing --non-interactive dist/*