fix tempfile.mkdtemp and shutil.rmtree for windows tests

Theres a linuxism in nikola code at
    utils.get_asset_path
when it suposses os.path.relpath will always succed.
That is false in windows, if paths are in different units
relpath will traceback.

Due to common settings in windows and mkdtemp MO, tests using
stock mkdtemp triggered always the bad linuxism case.

I wanted to make the related test cases to not trigger this
failure, so as to get more coverage before attempting to fix nikola.

So, this commit
 1. wraps tempfile.mkdtemp to ensure the result
    is in the same unit as the WC
 2. the resulting tempdir was getting a RO flag that prevented shutil.rmtree
    to delete the test directory, so added an error handler to fix that.
 3. unrelated, but minor: fix a trivial py2.6 format compat.

Before changes, nosetest* tells:
Ran 91 tests in 19.641s
FAILED (SKIP=4, errors=26, failures=12)
After changes:
noesetests tells:
Ran 91 tests in 37.515s
FAILED (SKIP=4, errors=11, failures=4)

* running without doctests because a lot of noise
  from trivial diference in os.sep
  also, some errors / failures because not full dependencies