contrib: Use asmap for ASN lookup in makeseeds

Add an argument `-a` to provide a asmap file to do the IP to ASN
lookups.

This speeds up the script greatly, and makes the output deterministic.
Also removes the dependency on `dns.lookup`.

I've annotated the output with ASxxxx comments to provide a way to
verify the functionality.

For now I've added instructions in README.md to download and use the
`demo.map` from the asmap repository. When we have some other mechanism
for distributing asmap files we could switch to that.

This continues #24824. I've removed all the fallbacks and extra
complexity, as everyone will be using the same instructions anyway.

Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>
Co-authored-by: James O'Beirne <james.obeirne@pm.me>
Co-authored-by: russeree <reese.russell@ymail.com>

contrib/seeds/README.md

@@ -8,21 +8,11 @@ and remove old versions as necessary (at a minimum when GetDesirableServiceFlags
 changes its default return value, as those are the services which seeds are added
 to addrman with).
 
-The seeds compiled into the release are created from sipa's DNS seed data, like this:
+The seeds compiled into the release are created from sipa's DNS seed and AS map
+data. Run the following commands from the `/contrib/seeds` directory:
 
     curl https://bitcoin.sipa.be/seeds.txt.gz | gzip -dc > seeds_main.txt
-    python3 makeseeds.py < seeds_main.txt > nodes_main.txt
+    curl https://bitcoin.sipa.be/asmap-filled.dat > asmap-filled.dat
+    python3 makeseeds.py -a asmap-filled.dat < seeds_main.txt > nodes_main.txt
     cat nodes_main_manual.txt >> nodes_main.txt
     python3 generate-seeds.py . > ../../src/chainparamsseeds.h
-
-## Dependencies
-
-Ubuntu, Debian:
-
-    sudo apt-get install python3-dnspython
-
-and/or for other operating systems:
-
-    pip install dnspython
-
-See https://dnspython.readthedocs.io/en/latest/installation.html for more information.