[Feature Request] TypeVar variance definition alias #813

Defining the two variances of a TypeVar is rather verbose:

T_co = TypeVar("T_co", covariant=True)
T_contra = TypeVar("T_contra", contravariant=True)

So I propose an alternative syntax, similar to that of Scala's variances:

T_co = +TypeVar("T_co")
T_contra = -TypeVar("T_contra")

Co- and contra-variance describe increasing and decreasing specificities of the typevars, respectively. So using + and - to symbolize these "directions" makes intuitive sense.