doctest blacklist dependency check

before blacklisting files check that their dependencies are not met

this PR 'unskips' a single doctest of the preview() function which does
not spawn a viewer
'py' is required for the doctests in conftest.py
partly fixes LaTeX doctest problem in galgebra/latex_ex
use import_module to check for dependencies
dependency check for doctests via a decorator
temporarily blacklist latex_ex until Issue 3969 is resolved
use @depends_on instead of blacklisting the preview.py doctest
use @depends_on in autowrap.py to check for f2py
create dummy executable to doctest functions which spawn a viewer
removed unused imports from preview.py
Monkey-patch pyglet for doctesting
doctest: module docstring dependency check support
add support for doctesting the plotting/pygletplot module
fix wrong output of some functions in the docstring of the pyplet module
fix test failures
disable doctesting of pyglet stuff, if no pyglet module is installed
workaround for class decorators in py2.5
fix doctesting of matplotlib stuff in plotting/plot_*.py
matplotlib is required to doctest plot_implicit
move depends_on import out of utilities/__init__.py