* LDAP import: python-ldap is not python3-compatible, pyldap is

  Warning: only supported from debian Stretch (current testing)?
  https://packages.debian.org/search?searchon=names&keywords=pyldap

* implicitly relative imports
* imports of moved or removed stdlib modules

issue #8530

Needed to import xlsx files

Closes #15644

In order to circumvent https://bugs.launchpad.net/lxml/+bug/1625506

closes #13721

The stdlib version of the json library is more recent than the 3.5.3
version we are pinning in `requirements.txt`

There is no reason to use it.

Closes #6940

The `unittest2` package is simply a backport of `unittest` from the
Standard Library of Python 2.7 to previous versions.

There is no reason to use it any longer.

Closes #6941

gevent 1.0.1 (old) causes issue on Debian 8.0 Jessie : Disk running out of free space
Cause: /var/log/syslog and /var/log/daemon.log full of "NameError: name 'PROTOCOL_SSLv3' is not defined"

FYI: disk of 20GB runs out of space in less than 24h!

gevent 1.0.2 release notes : https://github.com/gevent/gevent/blob/1.0.2/changelog.rst#release-102-may-23-2015

switch to reportlab 3.1.44 because of install problems with 3.1.8
(See https://bitbucket.org/rptlab/reportlab/issues/55/setuptools-80-cannot-install-reportlab-318 )

Workarounded https://launchpad.net/debian/+source/openssl/1.0.1j-1 by updating
requests in requirements.txt and pip in our tests environment.

This library is a SOAP client. It will be used to integrate Odoo with
external web services that use SOAP based protocols.

PyChart 1.39 is available on pypi.python.org, use it
in dependency specs instead of the direct link to
unreliable download.gna.org

Closes #6781

While calling `pip install -r requirements.txt`, the PyChart tarball is
always downloaded even if `pip` has a download cache configured (which
it does by default in later versions).

This is because `pip` has no way of knowing which package the PyChart
tarball contains before downloading and running its `setup.py`.

By declaring the PyChart "egg" in `requirements.txt` the tarball is no
longer downloaded if it's already in the download cache.

It is never used in the code, and causes problems with wheels: related
to pypa/pip#2105

closes #3744

[FIX] requirements.txt: add info about optional python requirements used for the point of sale receipt printing