Add missing is_readable method to AsyncSocket for httpx compatibility

structio/io/socket.py

@@ -1,3 +1,4 @@
+import select
 import warnings
 import platform
 from typing import Any
@@ -655,5 +656,20 @@ class AsyncSocket(AsyncResource):
                 except WantWrite:
                     await wait_writable(self._fd)
 
+    def is_readable(self):
+        """
+        Returns whether the OS thinks this socket is
+        readable. For more info see https://github.com/python-trio/trio/issues/760
+        """
+
+        match platform.system():
+            case "Windows":
+                readable, _, _ = select.select([self.socket], [], [], 0)
+                return bool(readable)
+            case _:
+                p = select.poll()
+                p.register(self.socket, select.POLLIN)
+                return bool(p.poll(0))
+
     def __repr__(self):
         return f"AsyncSocket({self.socket})"