BLD: Use NumPy nightly wheels for non-release builds

This uses the NumPy nightlies for non-release builds to ensure the
matplotlib nightlies uploaded are build against the new NumPy version.

These nightlies are not tested, but if they were they should actually
still be tested against the release NumPy.  This is necessary to allow
NumPy to enforce API changes, it does mean that potential C-API breaks
in NumPy need quicker follow-up unfortunately (either in NumPy or in
matplotlib).

Author: seberg

.github/workflows/cibuildwheel.yml

@@ -90,12 +90,35 @@ jobs:
     name: Build wheels on ${{ matrix.os }} for ${{ matrix.cibw_archs }}
     runs-on: ${{ matrix.os }}
     env:
-      CIBW_BEFORE_BUILD: >-
-        pip install numpy>=1.25 &&
-        rm -rf {package}/build
-      CIBW_BEFORE_BUILD_WINDOWS: >-
-        pip install delvewheel numpy>=1.25 &&
-        rm -rf {package}/build
+      # The following commands branch based on whether we are on the main
+      # branch or have a PR into main.  For these, we use the NumPy 2.0
+      # nightlies to ensure future C-API/ABI compatibility.
+      # This branching becomes unnecessary when NumPy 2.0 is released.
+      # When using no-build-isolation we need to install all requirements.
+      CIBW_BEFORE_BUILD:
+        ${{ (((github.event_name == 'push' && github.ref == 'refs/heads/main') ||
+              (github.event_name == 'pull_request' && github.base_ref == 'refs/heads/main')) &&
+            'pip install meson-python pybind11 setuptools_scm "setuptools>=64" ninja &&
+             pip install "numpy>=2.0.0.dev0" --extra-index-url "https://pypi.anaconda.org/scientific-python-nightly-wheels/simple" &&
+             rm -rf {package}/build'
+          ) ||
+            'pip install "numpy>=1.25" &&
+             rm -rf {package}/build'
+        }}
+      CIBW_BEFORE_BUILD_WINDOWS:
+        ${{ (((github.event_name == 'push' && github.ref == 'refs/heads/main') ||
+              (github.event_name == 'pull_request' && github.base_ref == 'refs/heads/main')) &&
+            'pip install delvewheel meson-python pybind11 setuptools_scm "setuptools>=64" ninja &&
+             pip install "numpy>=2.0.0.dev0" --extra-index-url "https://pypi.anaconda.org/scientific-python-nightly-wheels/simple" &&
+             rm -rf {package}/build'
+          ) ||
+            'pip install delvewheel "numpy>=1.25" &&
+             rm -rf {package}/build'
+        }}
+      CIBW_BUILD_FRONTEND: >-
+        ${{ (((github.event_name == '' && github.ref == 'refs/heads/main') ||
+              (github.event_name == 'pull_request' && github.base_ref == 'refs/heads/main')) &&
+            'pip; args: --no-build-isolation') || 'build' }}
       CIBW_REPAIR_WHEEL_COMMAND_WINDOWS: >-
         delvewheel repair -w {dest_dir} {wheel}
       CIBW_AFTER_BUILD: >-