requests: Allow passing None header values

python#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.

stubs/requests/requests/api.pyi

@@ -3,9 +3,7 @@ from collections.abc import Mapping
 from typing_extensions import TypeAlias
 
 from .models import Response
-from .sessions import RequestsCookieJar, _Auth, _Cert, _Data, _Files, _HooksInput, _Params, _TextMapping, _Timeout, _Verify
-
-_HeadersMapping: TypeAlias = Mapping[str, str | bytes]
+from .sessions import RequestsCookieJar, _Auth, _Cert, _Data, _Files, _HeadersUpdateMapping as _HeadersMapping, _HooksInput, _Params, _TextMapping, _Timeout, _Verify
 
 def request(
     method: str | bytes,