https://github.com/python/typeshed/pull/7773 changed
`requests.session.Session` methods to accept None for header values, but
didn't do quite the same for the functions in `requests.api`. I think
this was a mistake. The functions in `requests.api` just pass through
the `headers` argument without doing anything in particular to it.
Furthermore, it's useful to be able to pass None as a header value:
because `requests.utils.default_headers` sets an `Accept-Encoding`
header by default, the easiest way to send a request with no
`Accept-Encoding` header is something like `requests.get(url,
headers={"Accept-Encoding": None})`. It's annoying to have to construct
a `Session` just to pass type-checking.
It's a little confusing for the type alias to be called
`_HeadersUpdateMapping` in `requests.sessions` but `_HeadersMapping` in
`requests.api`; this is because the latter name was already used in
other type stubs (`tensorflow.keras.callbacks`), so it seemed best to
avoid breaking API.
