Adopt newer typing.cast workarounds from packaging

[pradyunsg]

See commits in pypa/packaging#291 for context.

[uranusjr]

Doesn’t pip._internal.utils.typing already has a smimlar setup? It does not use typing.TYPE_CHECKING (and it doesn’t need to; Mypy still type-check even if the constant is always false), but otherwise the same.

[pradyunsg]

It does; I actually copied it pip -> packaging. The issue is that the workaround for typing.cast is fragile and doesn't work in exactly the same way as typing.cast.

That got iterated upon there in that PR and we should bring those improvements back in.

[pradyunsg]

We don't need this anymore, thanks to dropping 2.7. :)