⬆️ UPGRADE: attrs -> v21 (#165)

This release has no breaking changes
(see: https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst)

Author: tadeu

setup.cfg

@@ -29,7 +29,7 @@ project_urls =
 [options]
 packages = find:
 install_requires =
-    attrs>=19,<21
+    attrs>=19,<22
     typing_extensions>=3.7.4;python_version<'3.8'
 python_requires = ~=3.6
 include_package_data = True