Tweak seekable check

Author: florimondmanca

httpx/_utils.py

@@ -326,7 +326,7 @@ def get_filelike_length(stream: typing.IO) -> int:
         fd = stream.fileno()
     except OSError:
         # No... is it something that supports random access, like `io.BytesIO`?
-        if not stream.seekable():
+        if not hasattr(stream, "seekable") or not stream.seekable():
             # Not even that? Sorry, we're doomed...
             raise  # pragma: nocover