debian: Remove dependency on python2 for libwally

Ubuntu Focal removed the python2 packages since that version is no longer
supported, and we can expect more distros to join soon.

debian/control

@@ -2,7 +2,7 @@ Source: lightningd
 Section: net
 Priority: optional
 Maintainer: Christian Decker <decker.christian@gmail.com>
-Build-Depends: debhelper (>=9),autotools-dev,build-essential,pkg-config,libsqlite3-dev,git,libtool,libgmp-dev,libsqlite3-dev,python,python3,net-tools,zlib1g-dev,python3-pip,autoconf,automake,python3-mako,gettext,libpq-dev
+Build-Depends: debhelper (>=9),autotools-dev,build-essential,pkg-config,libsqlite3-dev,git,libtool,libgmp-dev,libsqlite3-dev,python3,net-tools,zlib1g-dev,python3-pip,autoconf,automake,python3-mako,gettext,libpq-dev
 Depends: libsqlite3-0 bitcoind
 Standards-Version: 3.9.6
 Homepage: https://github.com/ElementsProject/lightning

debian/rules

@@ -24,7 +24,9 @@ override_dh_auto_build:
 # dh_make generated override targets
 # This is example for Cmake (See https://bugs.debian.org/641051 )
 override_dh_auto_configure:
-	./configure CC=gcc --disable-developer --disable-valgrind --prefix=/usr
+	# PYTHON_VERSION is required for libwally's ./configure script so it
+	# doesn't fall back on the deprecated (and removed) version 2
+	PYTHON_VERSION=3 ./configure CC=gcc --disable-developer --disable-valgrind --prefix=/usr
 
 override_dh_auto_test:
 	echo Skipping