[requests] Add _content to Response

[jermenkoo]

requests module defines _content as https://github.com/psf/requests/blob/8149e9fe54c36951290f198e90d83c8a0498289c/requests/models.py#L819-L836 and pyre was complaining about this missing

def foo(...) -> None:
    response = requests.Response()
    # pyre-fixme[16]: `Response` has no attribute `_content`.
    response._content = content

[srittau]

Technically, _content can also be False, but I'm fine with this.