py23 import in all nontrivial source files and unused import cleanup
diff --git a/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py b/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
index 57d896e..34326bc 100644
--- a/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
+++ b/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
@@ -2,6 +2,7 @@
# this class gets its own python file.
from fontTools.misc import sstruct
from fontTools.misc.textTools import safeEval
+from fontTools.misc.py23 import *
bigGlyphMetricsFormat = """