Write ligolw attributes in sorted order
The attributes are stored as a Python dictionary. Because
dictionaries in Python 3.6 and later preserve insertion order
and because ligolw_sqlite
has a slightly different code path
than most of the pure-XML tools, this makes XML files difficult to
reproduce byte-for-byte.
This fix will be needed for the unit tests to pass under Python 3.
Note: merge !6 (merged) first.
Edited by Leo P. Singer