__init__.py

bl_info = {
    "name": "Blender Toast",
    "description": "Toast Notifications",
    "author": "Josh Faul",
    "blender": (2, 90, 1),
    "category": "Render"
}

import subprocess
import os

import bpy
from bpy.app.handlers import persistent

@persistent
def notify(dummy):
    scr = r'& "{}\{}"'.format(os.path.dirname(__file__), "rendertoast.ps1")
    subprocess.run(['powershell.exe', scr])

def register():
    bpy.app.handlers.render_complete.append(notify)

def unregister():
    bpy.app.handlers.render_complete.remove(notify)

if __name__ == "__main__":
    register()

rendertoast.ps1

[void] [System.Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms")

$objNotifyIcon = New-Object System.Windows.Forms.NotifyIcon

$objNotifyIcon.Icon = [System.Drawing.SystemIcons]::Information
$objNotifyIcon.BalloonTipIcon = "Info"
$objNotifyIcon.BalloonTipTitle = "Render Done"
$objNotifyIcon.BalloonTipText = "The render. It is finished."
$objNotifyIcon.Visible = $True
$objNotifyIcon.ShowBalloonTip(10000)