CI: Ignore version duplicates when publishing to TestPyPI

This commit prevents the failure of the GitHub Actions job "Publish
distribution package to TestPyPI" on most pushes (see discussion at
#721 (comment)),
since recent commit a3e1325 "CI: Publish PyPI releases using GitHub
Actions workflows".

Indeed, TestPyPI doesn't allow repushing the same version. Use
`skip-existing` to prevent to such failures on the CI.

An alternative would be to use the `setuptools-scm` plugin to define a
unique version for each commit (e.g. `1.36.2.dev2+gd4f1c14`), but it
requires more changes (define the version in `pyproject.toml`, fetch
tags from GitHub in `publish.yaml`, exclude `.*` + `.github/**` files in
`MANIFEST.in`). Moreover it doesn't work as is: to prevent errors like
`HTTP 400 The use of local versions in '1.36.2.dev2+gd4f1c14' is not
allowed` we would need `local_scheme = "no-local-version"`, which would
push duplicated versions on TestPyPI anyway.

Author: adrienverge

.github/workflows/publish.yaml

@@ -46,6 +46,7 @@ jobs:
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
           repository-url: https://test.pypi.org/legacy/
+          skip-existing: true
 
   publish-to-pypi:
     name: Publish distribution package to PyPI