Bump attrs and cattrs dependencies (#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/airflow#11965

GitOrigin-RevId: 3ad037872e54ec617f1b2734781c61640c7528ca

setup.py

@@ -690,9 +690,11 @@ def is_package_excluded(package: str, exclusion_list: List[str]):
 INSTALL_REQUIREMENTS = [
     'alembic>=1.2, <2.0',
     'argcomplete~=1.10',
-    'attrs~=20.0',
+    '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',
     'connexion[swagger-ui,flask]>=2.6.0,<3',
     'croniter>=0.3.17, <0.4',