Rename local definitions in stubs (such as T) to use underscores (_T)

[JukkaL]

If a stub contains a local definition that should not be exported, it should be named with an underscore prefix so that from x import * does not import the definition.

For example, stubs/3.2/itertools.py defines these type variables:

T = typevar('T')
S = typevar('S')

It should be refactored like this:

_T = typevar('_T')
_S = typevar('_S')

(and all references to T and S should be fixed, of course)

No need to fix all of these in one go; this can be fixed gradually. The most important module is typing, since it would be nice to be able to use from typing import * without namespace pollution.