Not as strong as other languages perhaps, but Django & co. are more than capable of turning out nice web stuff.

Slightly OT.

As much as I love Python, in this case a '1-liner' using awk and sort or sed, tr, dd etc. would get you there 'faster'.

awk '{print tolower($0)}' domains.txt > whitelist.txt && cat whitelist.txt | sort -o whitelist.txt

That's probably just because the example is kept simple to illustrate Python's terseness, readability, productivity etc.