requests-oauthlib: add **kwargs to OAuth1Session.__init__

At runtime OAuth1Session.__init__ forwards **kwargs straight through to
OAuth1(...) (see requests_oauthlib/oauth1_session.py), which already has
extra keyword-only params typed in oauth1_auth.pyi (e.g. realm). The
OAuth1Session stub was missing the passthrough, so legitimate calls like
OAuth1Session(client_key=..., realm=...) failed with a spurious
"No parameter named realm" error. Sibling methods on the same class
(authorization_url, fetch_request_token, fetch_access_token) already use
a bare **kwargs for the same reason.

Verified with tests/stubtest_third_party.py, tests/mypy_test.py (all
supported Python versions), and pre-commit (ruff/black/flake8-pyi).

Author: zhukant

stubs/requests-oauthlib/requests_oauthlib/oauth1_session.pyi

@@ -51,6 +51,7 @@ class OAuth1Session(requests.Session):
         encoding: str = "utf-8",
         nonce=None,
         timestamp=None,
+        **kwargs,
     ) -> None: ...
 
     @property