Simplify CI

.gitlab-ci.yml

-image: clarinpl/python:3.8
+image: python:latest
 
-cache:
-  paths:
-    - .tox
 stages:
   - publish
 
 pypi_publish:
-  before_script:
-   - pip install build twine
-   - pip install urllib3==1.26.6
   only:
-   - main
+    - main
   stage: publish
   script:
+    - pip install build twine
     - python -m build
     - python -m twine upload
      --skip-existing