Start removing compatibility for Python < 2.6
Use floor division operator instead of function
Inline the effect of intbool() function
Remove code for Python < 2.2 in xlrd.xlsx
Add test for xlrd.formula : binary operation returning boolean
Remove unused BUFFER
Zoom into test sheet to exercise some more code.
Merge takluyver:modernise to get rid of a bunch of old code needed when xlrd supported Python < 2.6