Add PyPI publish workflow triggered on version tags

Uses PyPI Trusted Publisher (OIDC) - no API token needed.
Triggers on tags matching 'v*' (e.g. v1.15.0).
setuptools-scm derives the package version from the tag automatically.

To activate, configure a Trusted Publisher on PyPI:
  https://pypi.org/manage/project/mip/settings/publishing/
  - Owner: coin-or
  - Repository: python-mip
  - Workflow: publish.yml
  - Environment: pypi

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: h-g-s

.github/workflows/publish.yml

@@ -0,0 +1,53 @@
+name: Publish to PyPI
+
+on:
+  push:
+    tags:
+      - "v*"
+
+jobs:
+  build:
+    name: Build distribution
+    runs-on: ubuntu-24.04
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0  # needed for setuptools-scm to derive version from tags
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Install build
+        run: pip install build
+
+      - name: Build sdist and wheel
+        run: python -m build
+
+      - name: Upload dist artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: dist
+          path: dist/
+
+  publish:
+    name: Publish to PyPI
+    needs: build
+    runs-on: ubuntu-24.04
+    environment:
+      name: pypi
+      url: https://pypi.org/p/mip
+    permissions:
+      id-token: write  # required for OIDC trusted publishing
+
+    steps:
+      - name: Download dist artifacts
+        uses: actions/download-artifact@v4
+        with:
+          name: dist
+          path: dist/
+
+      - name: Publish to PyPI
+        uses: pypa/gh-action-pypi-publish@release/v1