py3: Fix combinat.designs module for python3 (final)

[vinklein]

Fix the remaining python3 errors in combinat.designs module :

• fix interface bewteen python3 and expat xml parser.
• replace some map usage with list comprehesion. The former map usage resulting in empty lists and therefore multiple errors.
• fix sorting issues in IncidenceStructure.__init__

Errors fixed :

sage -t --long src/sage/combinat/designs/ext_rep.py  # 11 doctests failed
sage -t --long src/sage/combinat/designs/bibd.py  # 6 doctests failed
sage -t --long src/sage/combinat/designs/difference_family.py  # 6 doctests failed
sage -t --long src/sage/combinat/designs/database.py  # 6 doctests failed

CC: @fchapoton

Component: python3

Author: Vincent Klein

Branch: 069a55f

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/26837

[vinklein]

Branch: u/vklein/26837

[vinklein]

New commits:

069a55f

Trac #26837: Fix combinat/designs module for python3:

[vinklein]

Commit: 069a55f

[vinklein]

Reviewer: chapoton

[vinklein]

Changed reviewer from chapoton to none

[fchapoton]

comment:5

green patchbot => positive review

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:6

ok

[vinklein]

comment:7

Merci!

[vbraun]

Changed branch from u/vklein/26837 to 069a55f

[jdemeyer]

comment:9

This change doesn't quite work:

-                self._points = sorted(points)
-                if self._points == list(range(len(points))) and all(isinstance(x,(int,Integer)) for x in self._points):
+                # if points are tuple, sort None before int types and str after int types
+                sortkey = lambda e: [(0 if x is None else 2 if isinstance(x, str) else 1, x) for x in e]\
+                    if isinstance(e, tuple) else e
+                self._points = sorted(points, key=sortkey)

See #26938.

[jdemeyer]

Changed commit from 069a55f to none