pyln.client: new functionality to access Gossmap.

It doesn't do much work, but it does parse the gossmap file and extract
nodes and channels.

[ Fixup by Michael Schmoock <michael@schmoock.net> ]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/tests/test_gossmap.py

@@ -0,0 +1,38 @@
+from pyln.client import Gossmap
+
+import os.path
+import lzma
+
+
+def unxz_data_tmp(src, tmp_path, dst, wmode):
+    fulldst = os.path.join(tmp_path, dst)
+    with open(fulldst, wmode) as out:
+        with lzma.open(os.path.join(os.path.dirname(__file__), "data", src), "rb")as f:
+            out.write(f.read())
+    return fulldst
+
+
+def test_gossmap(tmp_path):
+    sfile = unxz_data_tmp("gossip_store-part1.xz", tmp_path, "gossip_store", "xb")
+    g = Gossmap(sfile)
+
+    chans = len(g.channels)
+    nodes = len(g.nodes)
+
+    g.refresh()
+    assert chans == len(g.channels)
+    assert nodes == len(g.nodes)
+
+    # Now append.
+    unxz_data_tmp("gossip_store-part2.xz", tmp_path, "gossip_store", "ab")
+
+    g.refresh()
+
+    # It will notice the new ones.
+    assert chans < len(g.channels)
+    assert nodes < len(g.nodes)
+
+    # Whole load at the same time gives the same results.
+    g2 = Gossmap(sfile)
+    assert set(g.channels.keys()) == set(g2.channels.keys())
+    assert set(g.nodes.keys()) == set(g2.nodes.keys())