Fix tests and fixtures relying on expired pgp keys

PGP tests relied on expired gpg keys: upgrade with newer Debian keys from
https://ftp-master.debian.org/keys.html.
Download new fixtures files from http://ftp.debian.org/debian/dists/buster/

.flake8

@@ -0,0 +1,7 @@
+[flake8]
+max-line-length = 200
+ignore = E126,E241,E741,W504
+include =
+    system
+exclude =
+    system/env