win32/replace.py: Fix replacing items in files with UTF-8 content

win32/replace.py: Fix replacing items in files with UTF-8 content

Some files that this script will process might have UTF-8 items in
there, which can cause problems on Python 3.x as it is more strict and
careful on unicode issues.  Fix this by:

-Doing what we did before on Python 2.x
-Opening the file with encoding='utf-8' on Python 3.x