incompatibilities in retrieval.py for Windows systems

Reported by Marmaduke:

in retrieval.py, in get_from_zip:

• L139 w/ os.rename, fails if the targetname already exists, unlike on Unix, where the existing file simply ceases to exists, Windows refuses.
• L140 w/ os.remove, tries to remove the zipfile while Python still has the file open (from L128 with ZipFile context manager), which works on Unix, but not on Windows