[python] Add discontinue date for Python 3.6 (#7709)

This PR does a few things:
- Changes the existing warning message for Python 3.6 to print a discontinue date
- Will automatically fail new Python 3.6 deployments created after that date
- Consolidates logic to make Python version selection work in a similar manner to Node.js version selection
- Changes tests from JS to TS

packages/python/jest.config.js

@@ -0,0 +1,5 @@
+/** @type {import('@ts-jest/dist/types').InitialOptionsTsJest} */
+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+};