Update utils.pyi

Author: mrhx01

stubs/requests/requests/utils.pyi

@@ -49,7 +49,7 @@ def is_valid_cidr(string_network: str) -> bool: ...
 def set_environ(env_name: str, value: None) -> _GeneratorContextManager[None]: ...
 def should_bypass_proxies(url: _Uri, no_proxy: Iterable[str] | None) -> bool: ...
 def get_environ_proxies(url: _Uri, no_proxy: Iterable[str] | None = None) -> dict[Incomplete, Incomplete]: ...
-def select_proxy(url: _Uri, proxies: Mapping[str, str] | None) -> str: ...
+def select_proxy(url: _Uri, proxies: Mapping[str, str] | None) -> str | None: ...
 def resolve_proxies(
     request: Request | PreparedRequest, proxies: dict[str, str] | None, trust_env: bool = True
 ) -> dict[str, str]: ...