import webbrowser
webbrowser.open("file.txt")
Despite it’s name it will open in Notepad, gedit and so on. Never tried it but it’s said it works.
An alternative is to use
osCommandString = "notepad.exe file.txt"
os.system(osCommandString)
or as subprocess:
import subprocess as sp
programName = "notepad.exe"
fileName = "file.txt"
sp.Popen([programName, fileName])
but both these latter cases you will need to find the native text editor for the given operating system first.