It appears that a cross-platform file opening module does not yet exist, but you can rely on existing infrastructure of the popular systems. This snippet covers Windows, MacOS and Unix-like systems (Linux, FreeBSD, Solaris…):
import os, sys, subprocess
def open_file(filename):
if sys.platform == "win32":
os.startfile(filename)
else:
opener = "open" if sys.platform == "darwin" else "xdg-open"
subprocess.call([opener, filename])