Test emscripten workflow inspired by NumPy

Initial inspiration was taken from
numpy/numpy#24603.

Author: lagru

.github/workflows/emscripten.yml

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+# Copied from NumPy https://github.com/numpy/numpy/pull/24603
+# https://github.com/numpy/numpy/blob/328a9a488c3a146497be704d2cee2f9134e43b5b/.github/workflows/emscripten.yml
+#
+# To enable this workflow on a fork, comment out:
+#
+# if: github.repository == 'scikit-image/scikit-image'
+name: Test Emscripten/Pyodide build
+
+on:
+  pull_request:
+    branches:
+      - main
+      - maintenance/**
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read # to fetch code (actions/checkout)
+
+jobs:
+  build-wasm-emscripten:
+    runs-on: ubuntu-22.04
+    if: "github.repository == 'scikit-image/scikit-image'"
+    env:
+      PYODIDE_VERSION: 0.23.1
+      # PYTHON_VERSION and EMSCRIPTEN_VERSION are determined by PYODIDE_VERSION.
+      # The appropriate versions can be found in the Pyodide repodata.json
+      # "info" field, or in Makefile.envs:
+      # https://github.com/pyodide/pyodide/blob/main/Makefile.envs#L2
+      PYTHON_VERSION: 3.11.2
+      EMSCRIPTEN_VERSION: 3.1.32
+      NODE_VERSION: 18
+    steps:
+      - name: Checkout scikit-image
+        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
+
+      - name: set up python
+        id: setup-python
+        uses: actions/setup-python@61a6322f88396a6271a6ee3565807d608ecaddd1 # v4.7.0
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
+      - uses: mymindstorm/setup-emsdk@ab889da2abbcbb280f91ec4c215d3bb4f3a8f775 # v12
+        with:
+          version: ${{ env.EMSCRIPTEN_VERSION }}
+          actions-cache-folder: emsdk-cache
+
+      - name: Install pyodide-build
+        run: pip install "pydantic<2" build pyodide-build==$PYODIDE_VERSION
+
+      - name: Build
+        run: |
+          # Note that we need to cross-compile here. Using `pyodide build`
+          # fails on not having a cross file. The `source() function in
+          # `pyodide_build/cli/build.py` needs a --backend-flags parameter so
+          # that we can pass config-settings arguments to the backend.
+          # We'll then run into static assert issues in `pyconfig.h` due to:
+          # https://github.com/pyodide/pyodide/pull/2494#issuecomment-1118500743
+          # See also https://github.com/pyodide/pyodide/pull/2238 and the patches
+          # to _numpyconfig.h in https://github.com/pyodide/pyodide/tree/main/packages/numpy
+          #
+          # tl;dr this needs some work, and it seems like cross-compilation is
+          # hard because Pyodide doesn't ship its own Python interpreter that
+          # we can actually target during a direct cross build.
+          python -m build --wheel -Csetup-args=--cross-file=$PWD/tools/emscripten.meson.cross -Csetup-args=-Dallow-noblas=true
+
+      - name: set up node
+        uses: actions/setup-node@5e21ff4d9bc1a8cf6de233a3057d20ec6b3fb69d # v3.8.1
+        with:
+          node-version: ${{ env.NODE_VERSION }}
+
+      - name: Set up Pyodide virtual environment
+        run: |
+          pyodide venv .venv-pyodide
+          source .venv-pyodide/bin/activate
+          pip install dist/*.whl
+          python -c "import sys; print(sys.platform)"
+          pip install -r test_requirements.txt
+      - name: Test
+        run: |
+          source .venv-pyodide/bin/activate
+          cd ..
+          pytest --pyargs numpy -m "not slow"

tools/emscripten.meson.cross

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+# Copied from NumPy https://github.com/numpy/numpy/pull/24603
+# https://github.com/numpy/numpy/blob/328a9a488c3a146497be704d2cee2f9134e43b5b/tools/ci/emscripten.meson.cross
+
+[binaries]
+c = 'emcc'
+cpp = 'em++'
+ar = 'emar'
+fortran = '/usr/bin/gfortran'
+
+cmake = ['emmake', 'cmake']
+sdl2-config = ['emconfigure', 'sdl2-config']
+
+[host_machine]
+system = 'emscripten'
+cpu_family = 'wasm32'
+cpu = 'wasm'
+endian = 'little'
+
+[properties]
+longdouble_format = 'IEEE_QUAD_LE'