Bump attrs and cattrs dependencies (apache#11969)

`cattrs` now depends on `attrs >= 20.1.0`, because of `attr.resolve_types`.

Source: https://github.com/Tinche/cattrs/blob/master/HISTORY.rst#110-2020-10-29

closes apache#11965

Author: kaxil

setup.py

@@ -550,9 +550,11 @@ def write_version(filename=os.path.join(*[my_dir, "airflow", "git_version"])):
 INSTALL_REQUIREMENTS = [
     'alembic>=1.0, <2.0',
     'argcomplete~=1.10',
-    'attrs~=19.3',
+    'attrs>=20.0, <21.0',
     'cached_property~=1.5',
-    'cattrs~=1.0',
+    # cattrs >= 1.1.0 dropped support for Python 3.6
+    'cattrs>=1.0, <1.1.0;python_version<="3.6"',
+    'cattrs>=1.0, <2.0;python_version>"3.7"',
     'colorlog==4.0.2',
     'configparser>=3.5.0, <3.6.0',
     'croniter>=0.3.17, <0.4',