Fix remaining issues with GitHub Actions

[jni]

• [x] Docs deployment still hasn't been migrated
• [x] MacOS Py3.9 is failing for mysterious reasons — maybe install deps from conda?
• [x] Windows is failing in caching — also mysterious!

__________________________ test_path_length_caching ___________________________

    def test_path_length_caching():
        skeleton = Skeleton(skeleton3d)
        t0 = process_time()
        distances = skeleton.path_lengths()
        t1 = process_time()
        distances2 = skeleton.path_lengths()
        t2 = process_time()
>       assert t2 - t1 < t1 - t0
E       assert (15.84375 - 15.84375) < (15.84375 - 15.84375)

• [ ] Migrate coveralls from Travis to GHA and remove travis yml

[mkcor]

Docs deployment still hasn't been migrated

So, this means moving this functionality https://github.com/jni/skan/blob/97a217d36ec1393b380d4a797b5b7ceb68e824ec/.travis.yml#L46-L62 into .github/workflows/build-docs.yml