ltree_plpython failure test on Cygwin for 12.4 test

I am finally trying to move from python2.7 to python 3.x

for both 3.7 and 3.8 I have (attached log):

2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
ERROR:  incompatible library "/pub/devel/postgresql/prov
a38/postgresql-12.4-1.x86_64/build/tmp_install/usr/lib/postgresql/plpython3.dll": 
missing magic block
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
HINT:  Extension libraries are required to use the PG_MO
DULE_MAGIC macro.
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
STATEMENT:  CREATE EXTENSION ltree_plpython3u CASCADE;
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
ERROR:  language "plpython3u" does not exist
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
HINT:  Use CREATE EXTENSION to load the language into th
e database.
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython 
STATEMENT:  CREATE FUNCTION test1(val ltree) RETURNS int
         LANGUAGE plpython3u
         TRANSFORM FOR TYPE ltree
         AS $$
         plpy.info(repr(val))
         return len(val)
         $$;


Only the python tests fail

  $ grep FAIL postgresql-12.4-1-check.log
test python3/hstore_plpython      ... FAILED      423 ms
test python3/jsonb_plpython       ... FAILED      172 ms
test python3/ltree_plpython       ... FAILED      163 ms

never had problem with python2.7

Suggestion ?

Regards
Marco