fix: support pytest v6 (#307)

README.md

@@ -4,7 +4,7 @@
 
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-![Pytest>=5.1.0](https://img.shields.io/badge/pytest-%3E%3D5.1.0-green) [![Pypi](https://img.shields.io/pypi/v/syrupy)](https://pypi.org/project/syrupy/) [![Wheel](https://img.shields.io/pypi/wheel/syrupy)](https://pypi.org/project/syrupy/) ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/syrupy) [![PyPI - Downloads](https://img.shields.io/pypi/dm/syrupy)](https://pypi.org/project/syrupy/) [![PyPI - License](https://img.shields.io/pypi/l/syrupy)](./LICENSE)
+![Pytest>=5.1.0,<7.0.0](https://img.shields.io/badge/pytest-%3E%3D5.1.0,%20%3C7.0.0-green) [![Pypi](https://img.shields.io/pypi/v/syrupy)](https://pypi.org/project/syrupy/) [![Wheel](https://img.shields.io/pypi/wheel/syrupy)](https://pypi.org/project/syrupy/) ![PyPI - Python Version](https://img.shields.io/pypi/pyversions/syrupy) [![PyPI - Downloads](https://img.shields.io/pypi/dm/syrupy)](https://pypi.org/project/syrupy/) [![PyPI - License](https://img.shields.io/pypi/l/syrupy)](./LICENSE)
 
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