Specify how to annotate variable types

[JukkaL]

We should probably have a single convention for attribute and module-level variable annotations that would at least be recommended (even if other ways would also work).

Here are some potential approaches:

• x = 0, x = '', etc. (at least for built-in types)
• x = Undefined(int)
• x = Undefined # type: int
• # var x: int (a proposal for PEP 484, still under discussion)
• x = ClassName()

[JukkaL]

Another option:

• x = None # type: int (the type will be int, not Optional[int])

[JukkaL]

Yet another option:

• x = cast(int, None)

[vlasovskikh]

The PEP now allows specifying the type of a variable without specifying its value:

x = ...  # int

For variables with known default values we could use:

x = 42  # int

[gvanrossum]

The PEP picked a syntax. It is used in many stubs, and mypy supports it. So I think this issue is fixed.