Drop logilab/cubicweb:py35-stretch-dev

Debian package for dev version of cubicweb doesn't build on stretch due to missing python3-pycryptodome.
Now that buster is the new stable, we don't want to maintain this stretch "dev" image.

README.rst

@@ -18,7 +18,6 @@ Images with cubicweb pre-installed:
 * ``onbuild``, ``3.26-onbuild``, ``py37-buster-3.26-onbuild``
 * ``dev``, ``py37-buster-dev`` (built using latest mercurial changeset)
 * ``3.25``, ``py27-buster-3.25``
-* ``py35-stretch-dev``
 * ``py35-stretch-3.26``
 * ``py27-buster-3.26``
 * ``py27-stretch-3.25``

build.py

@@ -12,7 +12,7 @@ CWREPO = 'https://hg.logilab.org/master/cubicweb'
 MATRIX = [
     (['py27'], ['stretch', 'buster'], [None, '3.25', '3.26'],
         [None, 'onbuild']),
-    (['py35'], ['stretch'], [None, '3.26', 'dev'], [None, 'onbuild']),
+    (['py35'], ['stretch'], [None, '3.26'], [None, 'onbuild']),
     (['py37'], ['buster'], [None, '3.26', 'dev'], [None, 'onbuild']),
 ]
 ALIASES = (