Convert warning to warnings module
diff --git a/Lib/fontTools/ttLib/tables/_h_m_t_x.py b/Lib/fontTools/ttLib/tables/_h_m_t_x.py
index f7ea1e6..a17215f 100644
--- a/Lib/fontTools/ttLib/tables/_h_m_t_x.py
+++ b/Lib/fontTools/ttLib/tables/_h_m_t_x.py
@@ -31,7 +31,7 @@
 		if sys.byteorder != "big":
 			sideBearings.byteswap()
 		if data:
-			sys.stderr.write("too much data for hmtx/vmtx table\n")
+			warnings.warn("too much 'hmtx'/'vmtx' table data")
 		self.metrics = {}
 		for i in range(numberOfMetrics):
 			glyphName = ttFont.getGlyphName(i)