docs: update requirements, typo

- typo reported by @DavidHowlett, fixes #161
- update requirements and mention WinXP :(
diff --git a/serial/serialwin32.py b/serial/serialwin32.py
index 484c4a1..2927864 100644
--- a/serial/serialwin32.py
+++ b/serial/serialwin32.py
@@ -405,7 +405,7 @@
     def set_buffer_size(self, rx_size=4096, tx_size=None):
         """\
         Recommend a buffer size to the driver (device driver can ignore this
-        value). Must be called before the port is opended.
+        value). Must be called before the port is opened.
         """
         if tx_size is None:
             tx_size = rx_size