You can do that like this:
with open("/etc/apt/sources.list", "r") as sources:
lines = sources.readlines()
with open("/etc/apt/sources.list", "w") as sources:
for line in lines:
sources.write(re.sub(r'^# deb', 'deb', line))
The with statement ensures that the file is closed correctly, and re-opening the file in "w"
mode empties the file before you write to it. re.sub(pattern, replace, string) is the equivalent of s/pattern/replace/ in sed/perl.
Edit: fixed syntax in example