CLN: Fix imports, etc.

Remove wildcard import from test_merge and fixup

remove unnecessary imports from io/__init__

make _WINDOW_TYPES update 2/3 compatible
CLN: Py2/3 compatible __builtin__
CLN/ENH: Refactor url methods to be Py2/3 compatible
CLN: Use py2/3 compatible pickle import
CLN: py2/3 compat unichr, callable

CLN: py2/3 compatible unichr

CLN: Py2/3: callable --> six.callable

import fixes to wrap it all together

fixup erroneous find/replace
CLN: Py2/3-compatible dict keys/items/values.
more fixes