tests.yml: pypa/gh-action-pypi-publish has normalized inputs. Match it.

Solves two warnings:

Warning: Input 'skip_existing' has been deprecated with message: The inputs have been normalized to use kebab-case. Use  instead.
Warning: Input 'packages_dir' has been deprecated with message: The inputs have been normalized to use kebab-case. Use  instead.

Author: jamadden

.github/workflows/tests.yml

@@ -112,5 +112,5 @@ jobs:
         with:
           user: __token__
           password: ${{ secrets.TWINE_PASSWORD }}
-          skip_existing: true
-          packages_dir: wheelhouse/
+          skip-existing: true
+          packages-dir: wheelhouse/