You may use the free AutoHotkey.

The following example script will map F12 to your string:

F12::SendInput, alias python=python3.9

After installing AutoHotKey, put the above text in a .ahk file and double-click it to test. You may stop the script by right-click on the green H icon in the traybar and choosing Exit. To have it run on login, place it in the Startup group at
C:\Users\USER-NAME\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup.

Useful AutoHotkey documentation:

  • List of Keys
  • Hotkeys