IMAP.py removed warning about explicitly re-raising

This patch removes this warning:

repository/IMAP.py:146:16: W0707: Consider explicitly re-raising using the 'from' keyword (raise-missing-from)

offlineimap/repository/IMAP.py

@@ -147,7 +147,7 @@ class IMAPRepository(BaseRepository):
                     "remotehosteval option for repository "
                     "'%s' failed:\n%s" % (self, exc),
                     OfflineImapError.ERROR.REPO,
-                    exc_info()[2])
+                    exc_info()[2]) from exc
             if host:
                 self._host = host
                 return self._host