[BuildStream] Dropping support for Python 3.5 in master

[Jürg Billeter <j bitron ch>]

Hi everyone,

I propose we bump the required Python version in master to 3.6. Does
anyone see an issue with this?

We currently test Python 3.5 support with Debian 9. A recent docker
image build failure has made me wonder whether there is still a need to
support this.

All of the latest Linux LTS distro versions support Python 3.6 or
later. Ubuntu 18.04 LTS and SLES 15 include Python 3.6, Debian 10
(current stable) includes Python 3.7. RHEL/CentOS 7 support Python 3.6
via SCL (no Python 3 at all in the main repository). In RHEL/CentOS 8,
Python 3.6 is the default.

Upstream end-of-life of Python 3.5 is scheduled for 2020-09-13¹.

New features in Python 3.6 that could be useful:
* PEP 498: f-strings
* PEP 526: Type annotations for variables
* PEP 525: Asynchronous generators (and asyncio is now stable)
* PEP 519: File system path protocol

Also, `black` already requires Python 3.6.

This change will not affect BuildStream 1.x.

Cheers,
Jürg

¹ https://devguide.python.org/#status-of-python-branches