MAINT: weave: fix Python 3 syntax errors

The weave module doesn't work on Python 3, but these changes at least
make its syntax compatible, so that no SyntaxErrors arise on build.

2to3 fixers used: print, import, repr, exec, except
Added from __future__ import absolute_import, print_function
on top.
BUG: weave: remove dubious self-import causing problems with absolute imports