Raise reference Python to 3.6

[daniviga]

Meta-issue for Python 3.6 migration

ITEM	PR	STATUS	MERGE ORDER
oq-python-deb	gem/oq-python-deb#14	merged	1
oq-python-rpm	gem/oq-python-rpm#6	merged	1
oq-libs	gem/oq-libs#40	merged	2
env	gem/oq-builders#102	merged
Windows	gem/oq-builders#102	merged
opt	gem/oq-builders#102	merged
env	gem/oq-builders#102	merged
Docker	gem/oq-builders#103	merged	3
Engine	#3868	merged	4
VM	na	pending	5

Changes in oq-engine (#3868)

• Add deprecation warning for Python 3.5 in setup.py
• Add deprecation warning for Python 3.5 in openquake/commands/__main__.py

Documentation (#3868)

• Development install
• Other references to Python 3.5 and snippets

Other business

• oq-engine-celery

[micheles]

Note: one should remember to change openquake/commands/__main__.py which contains a check on the Python version (for instance we could print a deprecation message if Python 3.5 is used).

[daniviga]

Good catch @micheles, added

[daniviga]

Closing