u-u: import "email.genators" package early to workaround

This commit works around an issue when python itself is upgraded
by u-u and the content of email.message now needs a newer email.errors
but u-u already (implicitely) loaded that module so there is a
old version in sys.modules. So instead import email.generators early
so that it is available in memory and there is no mismatch.

See python/cpython#124170 and
https://bugs.launchpad.net/ubuntu/+source/python3.8/+bug/2080940
for details.

Thanks to Julian Klode for his excellent analysis.

Closes: LP:2080940

unattended-upgrade

@@ -28,6 +28,9 @@ import copy
 import datetime
 import errno
 import email.charset
+# workaround LP:2080940, email.errors changes as part of a security
+# update in python itself but is lazy loaded so import early to ensure
+import email.generator
 import fcntl
 import fnmatch
 import gettext