fix: Fix tests badge in README

Author: dairiki

README.md

@@ -1,6 +1,6 @@
 # marshmallow-dataclass
 
-[![Build Status](https://travis-ci.org/lovasoa/marshmallow_dataclass.svg?branch=master)](https://travis-ci.org/lovasoa/marshmallow_dataclass)
+[![Test Workflow Status (master branch)](https://img.shields.io/github/actions/workflow/status/lovasoa/marshmallow_dataclass/python-package.yml?branch=master&label=tests)](https://github.com/lovasoa/marshmallow_dataclass/actions/workflows/python-package.yml)
 [![PyPI version](https://badge.fury.io/py/marshmallow-dataclass.svg)](https://badge.fury.io/py/marshmallow-dataclass)
 [![marshmallow 3 compatible](https://badgen.net/badge/marshmallow/3)](https://marshmallow.readthedocs.io/en/latest/upgrading.html)
 [![download stats](https://img.shields.io/pypi/dm/marshmallow-dataclass.svg)](https://pypistats.org/packages/marshmallow-dataclass)