Remove macOS 13 from wheel targets and add macOS 15 Intel (#36)

GitHub announced that they are [retiring the macOS 13 runners next
month](https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/).
The images affected are:

- `macos-13`
- `macos-13-large`
- `macos-13-xlarge`

We may as well stop building them now. Doing so will not only prevent
workflows from soon failing: it will also speed up CI.

In addition, there are now `macos-15-intel` runners, for macOS on
x86_64, so this PR adds that as a new wheel build target.

Author: mhucka

.github/workflows/ci.yml

@@ -58,12 +58,6 @@ jobs:
           {os: ubuntu-24.04-arm, dist: cp313-manylinux_aarch64, arch: aarch64},
           {os: ubuntu-24.04-arm, dist: cp314-manylinux_aarch64, arch: aarch64},
 
-          {os: macos-13, dist: cp310-macosx_x86_64, arch: x86_64, target: '10.15'},
-          {os: macos-13, dist: cp311-macosx_x86_64, arch: x86_64, target: '10.15'},
-          {os: macos-13, dist: cp312-macosx_x86_64, arch: x86_64, target: '10.15'},
-          {os: macos-13, dist: cp313-macosx_x86_64, arch: x86_64, target: '10.15'},
-          {os: macos-13, dist: cp314-macosx_x86_64, arch: x86_64, target: '10.15'},
-
           {os: macos-14, dist: cp310-macosx_arm64, arch: arm64, target: '11.0'},
           {os: macos-14, dist: cp311-macosx_arm64, arch: arm64, target: '11.0'},
           {os: macos-14, dist: cp312-macosx_arm64, arch: arm64, target: '11.0'},
@@ -76,6 +70,12 @@ jobs:
           {os: macos-15, dist: cp313-macosx_arm64, arch: arm64, target: '11.0'},
           {os: macos-15, dist: cp314-macosx_arm64, arch: arm64, target: '11.0'},
 
+          {os: macos-15-intel, dist: cp310-macosx_x86_64, arch: x86_64, target: '11.0'},
+          {os: macos-15-intel, dist: cp311-macosx_x86_64, arch: x86_64, target: '11.0'},
+          {os: macos-15-intel, dist: cp312-macosx_x86_64, arch: x86_64, target: '11.0'},
+          {os: macos-15-intel, dist: cp313-macosx_x86_64, arch: x86_64, target: '11.0'},
+          {os: macos-15-intel, dist: cp314-macosx_x86_64, arch: x86_64, target: '11.0'},
+
           {os: macos-26, dist: cp310-macosx_arm64, arch: arm64, target: '11.0'},
           {os: macos-26, dist: cp311-macosx_arm64, arch: arm64, target: '11.0'},
           {os: macos-26, dist: cp312-macosx_arm64, arch: arm64, target: '11.0'},