Fix hacky imports causing clash with libplist

[hchargois]

The bare import plist in Scripts/plistwindow.py was supposed to import the plist.py module in the same (Scripts) package. However, if the libplist package was installed on the system, it actually imported the system's plist.so.

This kind of import (import X to import the module X in the same package) is called implicit relative import and according to PEP8, it should never be used. It isn't even supposed to work anymore in Python 3, but in this case, there was a hack to emulate it by adding the current package to the search path, making it a global import. However, the system packages took precedence.

I replaced it with a relative import.

I've tested that it works with both Python 2(.7) and 3(.8).

This should correctly fix #19, #22, #28, #31, without people having to remove their libplist system package.

[corpnewt]

Thanks for the explanation and PR on this.

-CorpNewt