Start testing against Python 3.9 (microsoft#14303)

Author: brettcannon

.github/workflows/insiders.yml

@@ -6,7 +6,7 @@ on:
  - main
 
 env:
- PYTHON_VERSION: 3.8
+ PYTHON_VERSION: 3.9
  MOCHA_REPORTER_JUNIT: true # Use the mocha-multi-reporters and send output to both console (spec) and JUnit (mocha-junit-reporter). Also enables a reporter which exits the process running the tests if it haven't already.
  CACHE_NPM_DEPS: cache-npm
  CACHE_OUT_DIRECTORY: cache-out-directory
@@ -142,7 +142,7 @@ jobs:
  # macOS runners are expensive, and we assume that Ubuntu is enough to cover the UNIX case.
  os: [ubuntu-latest, windows-latest]
  # Run the tests on the oldest and most recent versions of Python.
- python: [2.7, 3.8]
+ python: [2.7, 3.9]
  test-suite: [ts-unit, python-unit, venv, single-workspace, multi-workspace, debugger, functional]
  steps:
  - name: Checkout
@@ -372,7 +372,7 @@ jobs:
  # We're not running CI on macOS for now because it's one less matrix entry to lower the number of runners used,
  # macOS runners are expensive, and we assume that Ubuntu is enough to cover the UNIX case.
  os: [ubuntu-latest, windows-latest]
- python: [3.8]
+ python: [3.9]
  steps:
  # Need the source to have the tests available.
  - name: Checkout