u-u: reload the "email.errors" package 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.

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.

test/test_base.py

@@ -65,7 +65,7 @@ def make_fake_aptroot(self, template=None, fake_pkgs=[]):
         self.addCleanup(shutil.rmtree, tmpdir)
         aptroot = os.path.join(tmpdir, "aptroot")
         shutil.copytree(template, aptroot)
-        # apt warns if this dir does not exist
+        # apt warns if this dir does not existg
         os.makedirs(os.path.join(aptroot, "etc/apt/preferences.d"))
         # fake dpkg status
         mock_dpkg_status = os.path.join(aptroot, "var/lib/dpkg/status")

unattended-upgrade

@@ -28,6 +28,7 @@ import copy
 import datetime
 import errno
 import email.charset
+import email.errors
 import fcntl
 import fnmatch
 import gettext
@@ -40,6 +41,7 @@ import grp
 import inspect
 import io
 from importlib.abc import Loader
+import importlib
 import importlib.util
 import locale
 import logging
@@ -1630,6 +1632,9 @@ def _send_mail_using_sendmail(from_address, to_address, subject, body):
         [SENDMAIL_BINARY, "-oi", "-t"],
         stdin=subprocess.PIPE, universal_newlines=True)
     assert sendmail.stdin  # mypy
+    # workaround LP:2080940, email.errors changes as part of a security
+    # update in python itself and we need the new version
+    importlib.reload(email.errors)
     sendmail.stdin.write(msg.as_string())
     sendmail.stdin.close()
     ret = sendmail.wait()