refactor: use is_open instead of platform specific attributes, closes #83
diff --git a/serial/serialposix.py b/serial/serialposix.py
index c99dc0e..eff1009 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -712,7 +712,7 @@
         return less characters as requested. With no timeout it will block
         until the requested number of bytes is read.
         """
-        if self.fd is None:
+        if not self.is_open:
             raise portNotOpenError
         read = bytearray()
         poll = select.poll()