sdist build: bump base image to ubuntu 20.04

contrib/build-linux/sdist/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04@sha256:b58746c8a89938b8c9f5b77de3b8cf1fe78210c696ab03a1442e235eea65d84f
+FROM ubuntu:20.04@sha256:5747316366b8cc9e3021cd7286f42b2d6d81e3d743e2ab571f55bcd5df788cc8
 
 ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
 

contrib/build-linux/sdist/build.sh

@@ -9,6 +9,7 @@ DISTDIR="$PROJECT_ROOT/dist"
 
 . "$CONTRIB"/build_tools_util.sh
 
+# note that at least py3.7 is needed, to have https://bugs.python.org/issue30693
 python3 --version || fail "python interpreter not found"
 
 # upgrade to modern pip so that it knows the flags we need.