use os.sep instead of '/' for the pathname separator in import_file()
use os.sep instead of '/' for the pathname separator in import_file()

- fix portability issue

lokik/sfepy,RexFuzzle/sfepy,lokik/sfepy,RexFuzzle/sfepy,sfepy/sfepy,lokik/sfepy,BubuLK/sfepy,lokik/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,rc/sfepy,rc/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,vlukes/sfepy,vlukes/sfepy,BubuLK/sfepy,sfepy/sfepy