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