Regression in sip extract handling on Windows

[Scott Talbert]

Hi Phil,

With sip 6.7.4, I'm seeing this error when trying to build wxPython on 
Windows:

sip-build: an extract must be in the form 'id:file', not 
'pycode_core:D:\a\1\s\wx\core.py'

It looks like the new code splits on ':' and checks for len == 2, which is 
a problem for Windows paths.

Thanks,
Scott