Pydev tags import as “unresolved import” all compiled extensions - eclipse

Question

PyDev with Python3.5 seems unable to recognize imports from c-compiled extensions, including packages compiled via Cython.
I am working on an up-to-date debian/stretch machine with a stripped-down self-installed (in home dir) Eclipse/Neon with PyDev added via update site, if it matters.
I have both "Python 2.7.13" and "Python 3.5.2+" installed.
One of the problematic packages is lxml. I installed the debian packages and also tried installing manually via pip (and re-created the interpreter in eclipse afterwards to ensure full finding). In all cases packages do work.
In Python2 everything works as advertised.
Under Python3 PyDev flags from lxml import etree as error (but resulting program works):
Unresolved import: etree richiedi_certificato_dispositivo.py /trasmissione-telematica/Serializzazione line 8 PyDev Problem
Note: I can import lxml with no error, but then any access to lxml.etree... will be flagged in error. Data completion is consistent (i.e.: etree won't be in the offered list).
lxml is installed in the usual location:
mcon#vocore:~$ ls -l /usr/lib/python3/dist-packages/lxml
total 2156
-rw-r--r-- 1 root root 8152 Sep 5 2014 builder.py
-rw-r--r-- 1 root root 3366 May 5 2016 cssselect.py
-rw-r--r-- 1 root root 18387 May 5 2016 doctestcompare.py
-rw-r--r-- 1 root root 7641 Sep 25 2011 ElementInclude.py
-rw-r--r-- 1 root root 9490 Aug 20 06:48 _elementpath.py
-rw-r--r-- 1 root root 1710088 Aug 24 10:14 etree.cpython-35m-x86_64-linux-gnu.so
drwxr-xr-x 3 root root 4096 Jan 3 08:58 html
drwxr-xr-x 3 root root 4096 Jan 3 08:58 includes
-rw-r--r-- 1 root root 551 Oct 7 2012 __init__.py
drwxr-xr-x 4 root root 4096 Jan 3 08:58 isoschematron
-rw-r--r-- 1 root root 17450 Aug 20 06:48 lxml.etree_api.h
-rw-r--r-- 1 root root 8902 Aug 20 06:48 lxml.etree.h
-rw-r--r-- 1 root root 366440 Aug 24 10:14 objectify.cpython-35m-x86_64-linux-gnu.so
drwxr-xr-x 2 root root 4096 Jan 3 08:58 __pycache__
-rw-r--r-- 1 root root 92 Sep 5 2014 pyclasslookup.py
-rw-r--r-- 1 root root 8531 Nov 20 2014 sax.py
-rw-r--r-- 1 root root 230 Sep 25 2011 usedoctest.py
mcon#vocore:~/trasmissione-telematica$
As You can see etree is in a shared lib, as is objectify; a quick check shows also objectify is not handled by PyDev. I checked a few other "c-extension" packages (e.g.: import pycurl and from Crypto.Util import strxor) with the same result, so it seems a problem with "C" extensions.

Answer 1

Have you tried to put the offending packages in the forced builtins (as described in http://www.pydev.org/manual_101_interpreter.html)?
If that doesn't work, do you have some error in your error log? (see http://www.pydev.org/faq.html#PyDevFAQ-HowdoIReportaBUG%3F for details on getting it)