Update seeds tooling to Python 3

All the other tooling scripts require Python 3, it makes sense to do so
here too.

Also document the dependency on python3-dnspython.

contrib/seeds/README.md

@@ -1,4 +1,4 @@
-### Seeds ###
+# Seeds
 
 Utility to generate the seeds.txt list that is compiled into the client
 (see [src/chainparamsseeds.h](/src/chainparamsseeds.h) and other utilities in [contrib/seeds](/contrib/seeds)).
@@ -6,6 +6,11 @@ Utility to generate the seeds.txt list that is compiled into the client
 The seeds compiled into the release are created from sipa's DNS seed data, like this:
 
     curl -s http://bitcoin.sipa.be/seeds.txt > seeds_main.txt
-    python makeseeds.py < seeds_main.txt > nodes_main.txt
-    python generate-seeds.py . > ../../src/chainparamsseeds.h
+    python3 makeseeds.py < seeds_main.txt > nodes_main.txt
+    python3 generate-seeds.py . > ../../src/chainparamsseeds.h
 
+## Dependencies
+
+Ubuntu:
+
+    sudo apt-get install python3-dnspython