Clarify the version of astroid which is supported

This doesn't preclude supporting other versions at some point,
however it will make it clearer which versions are supported
as well as making CI pass.

setup.cfg

@@ -40,7 +40,13 @@ install_requires =
 setup_requires = setuptools>=44; setuptools_scm[toml]>=3.4.3
 
 [options.extras_require]
-test = astroid; pytest
+astroid =
+    astroid >=1, <2; python_version < "3"
+    astroid >=2, <3; python_version >= "3"
+test =
+    astroid >=1, <2; python_version < "3"
+    astroid >=2, <3; python_version >= "3"
+    pytest
 
 [options.package_data]
 asttokens = py.typed