Update contrib/seeds scripts to use the new .format() method

Summary: Avoid using % to format strings, switch to the new method instead.

Test Plan:
Run a bitcoin seeder instance for at least 100s in order to get a
dnsseed.dump file. From a build directory:

./src/bitcoin-seeder

(wait 100s then kill it)

mv dnsseed.dump ../contrib/seeds/seeds_main.txt
cd ../contrib/seeds
./makeseeds.py < seeds_main.txt > nodes_main.txt
./generate-seeds.py . > chainparamsseeds.h
cat chainparamsseeds.h

Ensure the chainparamsseeds.h file is correctly formatted.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2573