Add support for Android/iOS wheels

[hajimes]

This PR adds support for Android / iOS wheels, thanks to the excellent updates of cibuildwheel 3.0.0 and 3.1.0 (specifically, PR #2349 for Android and PR #2286 for iOS).

Updating the GitHub Actions workflow to accommodate these platforms was a bit challenging, so I'm sharing my experience here. I hope it may help other engineers in the future, when they face similar issues. My own code is available at https://github.com/hajimes/mmh3/blob/master/.github/workflows/wheels.yml.

1. Start with the official documentation
   See the cibuildwheel platforms and options guides:

   • https://cibuildwheel.pypa.io/en/stable/platforms/
   • https://cibuildwheel.pypa.io/en/stable/options/

   Also, consult their YAML example:

   • https://github.com/pypa/cibuildwheel/blob/main/examples/github-deploy.yml

2. Explicitly declare platform (usually in the build matrix)
   For standard platforms (linux, windows, macos), cibuildwheel infers this automatically from runner's OS. But for android and ios, you must declare it manually. Then, set CIBW_PLATFORM: ${{ matrix.platform || 'auto' }} to ensure proper platform selection. This instructs cibuildwheel to use the platform value specified in the build matrix when available; otherwise, it falls back to the runner's OS.

3. Configure Android/iOS-specific environment variables for testing
   For example, if using pytest and your tests are under ./tests, define:

   CIBW_TEST_SOURCES_ANDROID: "./tests"
   CIBW_TEST_SOURCES_IOS: "./tests"
   CIBW_TEST_COMMAND_ANDROID: "python -m pytest ./tests"
   CIBW_TEST_COMMAND_IOS: "python -m pytest ./tests"

   Standard platforms usually use pytest {project} as the testing command with cibuildwheel. However, Android/iOS builds cannot resolve the {project} placeholder, so explicit paths are required. Also note: CIBW_TEST_SOURCES_* must be defined, even if test file paths are stated within CIBW_TEST_COMMAND_*.

4. For Android-Intel builds (that is, *-android_x86_64), use a Linux runner (e.g., ubuntu-latest) and enable the kernel-based virtual machine (KVM) for testing.
   If you don't specify this, testing will fail (as of cibuildwheel 3.1.1). You can just copy and paste the code from https://github.blog/changelog/2024-04-02-github-actions-hardware-accelerated-android-virtualization-now-available/. Also, see https://github.com/pypa/cibuildwheel/blob/v3.1.1/examples/github-deploy.yml#L55-L61 for the real-world example.

5. For Android-ARM builds (that is, *-android_arm64_v8a), the canonical approach is to use an Apple Silicon runner (e.g., macos-latest) and skip testing.
   As of cibuildwheel 3.1.1, macOS runners on GitHub Actions do not support testing under Android emulation. Therefore, you must explicitly skip tests using CIBW_TEST_SKIP: "*-android_arm64_v8a". Alternatively, you can build on Intel Linux (e.g., ubuntu-latest), where tests are automatically skipped due to emulator limitations (no need to set CIBW_TEST_SKIP). This results in one fewer configuration line. However, from a code maintanability perspective, I recommend using macos for future extensions and support.

6. For iOS builds,
   Use:

   • macos-13 (Intel Mac runner) for x86_64_iphonesimulator
   • macos-14or macos-latest (Apple Silicon runners) for arm64_iphoneos and arm64_iphonesimulator architectures