Specify attrs minimum required version

We use kw_only in attr.s which was added in 18.2.0 [1]

https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst#1820-2018-09-01

Author: albertvaka

setup.cfg

@@ -28,7 +28,7 @@ setup_requires =
   setuptools >= 40.6
   pip >= 10
 install_requires =
-  attrs
+  attrs >= 18.2.0
   typing-extensions
 
 [options.packages.find]