Remove unnecessary optionality for requests.Session (#1658)
Remove unnecessary optionality for requests.Session (#1658)

Current code that initializes the things I've changed, to demonstrate that the
object is initialized this way:

* headers: https://github.com/requests/requests/blob/9713289e74/requests/sessions.py#L345
* proxies: https://github.com/requests/requests/blob/9713289e74/requests/sessions.py#L354
* hooks: https://github.com/requests/requests/blob/9713289e74/requests/sessions.py#L357
* params: https://github.com/requests/requests/blob/9713289e74/requests/sessions.py#L362
* cookies: https://github.com/requests/requests/blob/9713289e74/requests/sessions.py#L388

from what I can tell nothing in the official requests API ever creates a
session other than through `__init__`.
li-dan/typeshed,euresti/typeshed