pyln: Add psycopg2-binary as a dependency

Using the psycopg2-binary package means that the apropriate compiled binary
for the user platform will be shipped alongside the python binaries. Otherwise
the python bindings and the C shims would be shipped which would then require
the postgres development packages as well.

This just makes things easier, since we don't require the build dependencies.

contrib/pyln-testing/requirements.txt

@@ -3,3 +3,4 @@ Flask==1.1.1
 cheroot==6.5.5
 ephemeral-port-reserve==1.1.1
 python-bitcoinlib==0.10.1
+psycopg2-binary==2.8.3