Remove miscellaneous workarounds for pre-2.6 Python

* nested scopes enabled in 2.2
* bool() added in 2.3
* “int” is a type object since 2.2
Remove ZipFile.open() workaround for pre-2.6 Python
Use exception handler target variables rather than sys.exc_info()

In Python 3, saving the exception object manually will create a reference
cycle via the traceback attribute. Using the exception handler method means
the variable is automatically deleted, avoiding the reference cycle.
Merge branch 'py2.6' into py3-later

Conflicts:
	xlrd/book.py