Add pixi support

.gitattributes

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+# GitHub syntax highlighting
+pixi.lock linguist-language=YAML
+

.github/workflows/macos-linux-windows-pixi.yml

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+name: CI - MacOS/Linux/Windows via Pixi
+
+on:
+  push:
+    paths-ignore:
+      - 'doc/**'
+      - '.gitlab-ci.yml'
+      - '.gitignore'
+      - '*.md'
+      - 'LICENSE'
+      - 'colcon.pkg'
+      - '.pre-commit-config.yaml'
+  pull_request:
+    paths-ignore:
+      - 'doc/**'
+      - '.gitlab-ci.yml'
+      - '.gitignore'
+      - '*.md'
+      - 'LICENSE'
+      - 'colcon.pkg'
+      - '.pre-commit-config.yaml'
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  eigenpy-pixi:
+    name: ${{ matrix.os }} - Env ${{ matrix.environment }} ${{ matrix.build_type }} ${{ matrix.cxx_options }} ${{ matrix.compiler }}
+    runs-on: ${{ matrix.os }}
+    env:
+      CCACHE_BASEDIR: "${GITHUB_WORKSPACE}"
+      CCACHE_DIR: "${GITHUB_WORKSPACE}/.ccache"
+      CCACHE_COMPRESS: true
+      CCACHE_COMPRESSLEVEL: 6
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, macos-13]
+        environment: [all, all-py39]
+        build_type: [Release, Debug]
+        cxx_options: ['', '-mavx2']
+
+        exclude:
+          - os: macos-latest
+            cxx_options: '-mavx2'
+          - os: macos-13
+            cxx_options: '-mavx2'
+
+        include:
+          - os: windows-latest
+            environment: all
+            cxx_options: ''
+            build_type: Release
+          - os: windows-latest
+            environment: all-clang-cl
+            cxx_options: ''
+            build_type: Release
+
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - uses: actions/cache@v4
+      with:
+        path: .ccache
+        key: ccache-macos-linux-windows-pixi-${{ matrix.os }}-${{ matrix.build_type }}-${{ matrix.cxx_options }}-${{ matrix.environment }}-${{ github.sha }}
+        restore-keys: ccache-macos-linux-windows-pixi-${{ matrix.os }}-${{ matrix.build_type }}-${{ matrix.cxx_options }}-${{ matrix.environment }}-
+
+    - uses: prefix-dev/[email protected]
+      with:
+        pixi-version: v0.31.0
+        cache: true
+        environments: ${{ matrix.environment }}
+
+    - name: Build EigenPy [MacOS/Linux/Windows]
+      env:
+        CMAKE_BUILD_PARALLEL_LEVEL: 2
+        EIGENPY_BUILD_TYPE: ${{ matrix.build_type }}
+        EIGENPY_CXX_FLAGS: ${{ matrix.cxx_options }}
+      run: |
+        pixi run -e ${{ matrix.environment }} test
+
+  check:
+    if: always()
+    name: check-macos-linux-windows-pixi
+
+    needs:
+    - eigenpy-pixi
+
+    runs-on: Ubuntu-latest
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}

.github/workflows/update_pixi_lockfile.yml

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+name: CI - Update Pixi lockfile
+permissions:
+  contents: write
+  pull-requests: write
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: 0 5 1 * *
+
+jobs:
+  pixi-update:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/create-github-app-token@v1
+        id: generate-token
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
+      - uses: actions/checkout@v4
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          ref: devel
+          # Make sure the value of GITHUB_TOKEN will not be persisted in repo's config
+          persist-credentials: false
+
+      - name: Set up pixi
+        uses: prefix-dev/[email protected]
+        with:
+          run-install: false
+
+      - name: Update lockfile
+        run: |
+          set -o pipefail
+          pixi update --json | pixi exec pixi-diff-to-markdown >> diff.md
+
+      - name: Create pull request
+        uses: peter-evans/create-pull-request@v7
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          commit-message: 'pixi: Update pixi lockfile'
+          title: Update pixi lockfile
+          body-path: diff.md
+          branch: topic/update-pixi
+          base: devel
+          labels: pixi
+          delete-branch: true
+          add-paths: pixi.lock