Feature request: use consistent (ideally, portable) fileseps on Windows

[bersbersbers]

On Windows, pyi-makespec --splash sub/x.png sub/x.py outputs:

a = Analysis(
    ['sub\\x.py'],
    ...

splash = Splash(
    'sub/x.png',
    ...

1. File system separators are not consistent: "\\" != "/"
2. File system separators are not portable: this .spec file cannot be used on Linux.

I suggest that all fileseps be replaced by /, which should work on all OSes.

[rokm]

That's a good point - especially the non-portability of the .spec.

We could probably re-purpose the existing escape_win_filepath to replace the \ with / instead of \\.

This is also a good opportunity to fix the .spec templates to use %r for string formatting in general, as I suspect unpleasant happen if a string (a path, or application name, or anything really) contains an '.