Python 3.6 added a new `policy` attribute to `MIMEMultipart`.
MIMEMultipart may be part of the cached object pickle of a datasource.
Upgrading from an old version of python to 3.6+ will cause the
datasource to be invalid after pickle load.

This commit uses the upgrade framework to attempt to access the mime
message and fail early (thus discarding the cache) if we cannot.
Commit 78e89b0 should fix this issue more generally.