Add explicit dependency on attrs

Fix #77

Author: bluetech

setup.py

@@ -21,7 +21,7 @@
     use_scm_version=True,
     setup_requires=["setuptools-scm", "setuptools>=40.0"],
     python_requires=">=3.7",
-    install_requires=["pytest>=7.0"],
+    install_requires=["pytest>=7.0", "attrs>=attrs>=19.2.0"],
     classifiers=[
         "Development Status :: 4 - Beta",
         "Framework :: Pytest",