Fix get_full_context() finding filename path

[kr1zo]

Issue #55

[okken]

Tried it. But I just don't like the long paths. Relative is way handier for me.
A workaround for the problem is to create a venv in the test directory, then everything will be on D.
At least, I think that will work.

[rloutrel]

@okken : Can we consider, it is not expected that os.path return an absolute path when calling relpath and the current behvior is not really a bug, but that the function caller should handle the exception?

May I work on a fix for this where the exception is caught and fallback in abspath, like:

try:    
    filename = os.path.relpath(filename)
except ValueError as error:
    filename = os.path.abspath(filename)

it would produce the long path ONLY when the relative path is not possible to be built (like on wind**s system with multiple drives)