Update Python to 3.8.8 (#15278)

* update python fwlinks and remove bundle ver

* start fixing path for users with python 36

* prompt user for python version upgrade

* update python path after removing 3.6

* prompt users to upgrade and show pkg warning

* make prompt async

* remove python bundle ver from ML extension

* shutdown python 3.6 before deleting

* check useExistingPython before update prompt

* add dont ask again option

* remove 3.6 after installing 3.8

fix merge conflict

* give option to remove python36

* list user installed pip packages in warning

* create notebook to install pip packages

* update getPythonExePath method and add comments

* clean up code

* add comments

* pr comments

* add comment

* remove option to keep python36

* shutdown active servers before removing python36

* fix error removing old python w/ path change

* update to 3.8.10

* restart sessions for mac/linux

extensions/notebook/package.json

@@ -38,6 +38,11 @@
           "default": false,
           "description": "%notebook.useExistingPython.description%"
         },
+        "notebook.dontPromptPythonUpdate": {
+          "type": "boolean",
+          "default": false,
+          "description": "%notebook.dontPromptPythonUpdate.description%"
+        },
         "notebook.overrideEditorTheming": {
           "type": "boolean",
           "default": true,