Update badges

Remove appveyor and add GitHub workflows

Closes #41

Author: jeromekelleher

README.md

@@ -1,5 +1,5 @@
 # tszip
-[![CircleCI](https://circleci.com/gh/tskit-dev/tszip.svg?style=svg)](https://circleci.com/gh/tskit-dev/tszip) [![Build status](https://ci.appveyor.com/api/projects/status/792u70f5yndxcpes?svg=true)](https://ci.appveyor.com/project/tskit-dev/tszip) [![codecov](https://codecov.io/gh/tskit-dev/tszip/branch/main/graph/badge.svg)](https://codecov.io/gh/tskit-dev/tszip) [![Documentation Status](https://readthedocs.org/projects/tszip/badge/?version=latest)](https://tszip.readthedocs.io/en/latest/?badge=latest)
+[![Tests](https://github.com/tskit-dev/tszip/actions/workflows/tests.yml/badge.svg)](https://github.com/tskit-dev/tszip/actions/workflows/tests.yml) [![CircleCI](https://circleci.com/gh/tskit-dev/tszip.svg?style=svg)](https://circleci.com/gh/tskit-dev/tszip) [![codecov](https://codecov.io/gh/tskit-dev/tszip/branch/main/graph/badge.svg)](https://codecov.io/gh/tskit-dev/tszip) [![Documentation Status](https://readthedocs.org/projects/tszip/badge/?version=latest)](https://tszip.readthedocs.io/en/latest/?badge=latest)
 
 Gzip-like compression for [tskit](https://tskit.readthedocs.io/) tree sequences.