Add urllib.request.getproxies_environment() (#12524)

stdlib/urllib/request.pyi

@@ -79,6 +79,7 @@ else:
     def pathname2url(pathname: str) -> str: ...
 
 def getproxies() -> dict[str, str]: ...
+def getproxies_environment() -> dict[str, str]: ...
 def parse_http_list(s: str) -> list[str]: ...
 def parse_keqv_list(l: list[str]) -> dict[str, str]: ...