Install python3-attr from buster-backports

Otherwise, Electrum aborts on startup with an exception:

    ValueError: Non keyword-only attributes are not allowed after
    a keyword-only attribute.

I've reported to Debian the fact this dependency should be versioned:
https://bugs.debian.org/968563

refs #17828

config/chroot_apt/preferences

@@ -12,7 +12,7 @@ Pin: release o=Debian,n=sid
 Pin-Priority: 999
 
 Explanation: Electrum needs a version newer than the one in Buster
-Package: python3-protobuf libprotobuf23
+Package: python3-attr python3-protobuf libprotobuf23
 Pin: release o=Debian,n=buster-backports
 Pin-Priority: 990