Drop Python 2.7 support.

[dHannasch]

With PyYAML 5.4 released as of recently, we could resolve #476 if we wanted.

[hugovk]

As a minimum also remove these lines:

pyyaml/setup.py

Lines 30 to 31 in ee37f46

	"Programming Language :: Python :: 2",
	"Programming Language :: Python :: 2.7",

Consider adding "Programming Language :: Python :: 3 :: Only"

And update:

pyyaml/setup.py

Line 295 in ee37f46

python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*',

to:

python_requires='>=3.6', 

[Thom1729]

Superseded by #550.