exchange: extract a function to sort obsolete markers Follows up on changeset 01e8eefd9434, several other occurrences of sorted() need to be fixed. The _sortedmarkers() handles sorting obsmarkers with a None value as last element on Python 3.

py3: encode underlying error message during parse error of %include

convert: convert os.devnull to bytes before trying to join it with other bytes Together with the previous commit relating to emailparser this gets test-convert-tla.t passing under py3. Differential Revision: https://phab.mercurial-scm.org/D7062