fix(python): update cattrs dependency specification (#2212)

`cattrs>=1.1.0` dropped support for python 3.6, and added support for
python 3.9. Consequently, declare a dependency on `cattrs~=1.0.0` when
the python runtime is `< 3.7`, and on `cattrs~=1.1.0` when the python
runtime is `>= 3.7` in order to get the correct dependency brought in.

Fixes aws/aws-cdk#11219

Caused by Tinche/cattrs@eb454d4

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/@jsii/python-runtime/setup.py

@@ -31,12 +31,13 @@
     },
     install_requires=[
         "attrs~=20.1",
-        "cattrs~=1.0",
+        "cattrs~=1.0.0 ; python_version < '3.7'",
+        "cattrs~=1.1.0 ; python_version >= '3.7'",
         "importlib_resources ; python_version < '3.7'",
         "python-dateutil",
         "typing_extensions~=3.7",
     ],
-    python_requires=">=3.6",
+    python_requires="~=3.6",
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",