Log Message:
Update py-xlrd to 0.9.3.

This release includes the following changes:

- Github issue #49

- Github issue #64 - skip meaningless chunk of 4 zero bytes between two
  otherwise-valid BIFF records

- Github issue #61 - fix updating of escapement attribute of Font
  objects read from workbooks.

- Implemented Sheet.visibility for xlsx files

- Ignore anchors ($) in cell references

- Dropped support for Python 2.5 and earlier, Python 2.6 is now the
  earliest Python release supported

- Read xlsx merged cell elements.

- Read cell comments in .xlsx files

- Added xldate_to_datetime() function to convert from Excel
  serial date/time to datetime.datetime object.