u-u: import "email" package late to workaround (LP: 2080940)

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 as late as
possible.

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.

unattended-upgrade

@@ -27,7 +27,6 @@ import contextlib
 import copy
 import datetime
 import errno
-import email.charset
 import fcntl
 import fnmatch
 import gettext
@@ -40,6 +39,7 @@ import grp
 import inspect
 import io
 from importlib.abc import Loader
+import importlib
 import importlib.util
 import locale
 import logging
@@ -71,7 +71,6 @@ except ImportError:
 
 from collections import defaultdict, namedtuple
 from datetime import date
-from email.message import Message
 from gettext import gettext as _
 from io import StringIO
 from optparse import (
@@ -1613,9 +1612,21 @@ def _send_mail_using_mailx(from_address, to_address, subject, body):
 
 
 def _send_mail_using_sendmail(from_address, to_address, subject, body):
+    # workaround LP:2080940, email.errors changes as part of a security
+    # update in python itself so import this as late as possible
+    import email
+    import email.charset
+    import email.errors
+    importlib.reload(email.errors)    
+    import email.generator
+    import email.message
+    importlib.reload(email)
+    importlib.reload(email.charset)
+    importlib.reload(email.message)
+    importlib.reload(email.generator)
     # type: (str, str, str, str) -> int
     # format as a proper mail
-    msg = Message()
+    msg = email.message.Message()
     msg['Subject'] = subject
     msg['From'] = from_address
     msg['To'] = to_address