Drop support for Python 3.4 (and 3.5?) for Pyramid 2.0?

[stevepiercy]

This issue is a placeholder for discussing the drop of support for Python 3.4 and 3.5.

• Python 3.4 will be EOLed on 2019-03-16.
• Python 3.5 will be EOLed on 2020-09-13.

See Status of Python branches in the Python Developer's Guide.

What features in Python >= 3.6 could we implement by dropping either of these earlier versions? Please comment below.

See also #2362

[digitalresistor]

Type annotations please. f strings may be worth it too... maybe.

[hugovk]

Here's the pip installs for Pyramid from PyPI for December 2018:

category	percent	downloads
2.7	47.89%	49,041
3.6	32.60%	33,379
3.5	9.08%	9,299
3.7	5.37%	5,498
3.4	4.11%	4,204
null	0.60%	610
2.6	0.18%	185
3.3	0.18%	182
3.8	0.01%	6
Total		102,404

Source: pypistats python_minor --last-month pyramid

[ztane]

Type annotations are backportable. I wouldn't drop 3.5 before the EOL date for sure because there are many server OSes in existence where the 3.5 is the default

[mmerickel]

We need to drop 3.4 for sure.

[mmerickel]

fixed by #3547