Add arm64 Apple Mac M1 architecture to the build matrix

I'd like to use SPlisHSPlasH on Mac M1. Looks like there is no package for Mac M1 arm64 architecture.

```
pip install pysplishsplash
Defaulting to user installation because normal site-packages is not writeable
ERROR: Could not find a version that satisfies the requirement pysplishsplash (from versions: none)
ERROR: No matching distribution found for pysplishsplash
```