Add 'publish' stage to .gitlab-ci.yml

We add a new 'publish' stage in .gitlab-ci.yml to handle publishing artifacts.

.gitlab-ci.yml

@@ -2,6 +2,7 @@ image: python:3.10
 
 stages:
   - test
+  - publish
 
 before_script:
   - python -V
@@ -9,9 +10,19 @@ before_script:
   - source venv/bin/activate
   - pip install -U pip
   - pip install .
-  - echo "[CarbonTracer]" > config.ini
-  - echo "key = ${API_KEY}" >> config.ini
 
 test:
   stage: test
-  script: python -m unittest test.py
+  script: 
+    - echo "[CarbonTracer]" > config.ini
+    - echo "key = ${API_KEY}" >> config.ini
+    - python -m unittest test.py
+
+publish:
+  stage: publish
+  script:
+    - pip install -U hatchling twine build
+    - python -m build .
+    - python -m twine upload --username __token__ --password ${PYPI_TOKEN} dist/*
+  only:
+    - tags