Fix lyxpak to be Python 3 compatible

[José Matos]

Based on the error reported on the lyx-users here follows a fix for
issue reported.

I took the change and I changed some minor annoyances:
* test comparison with None;
* fixed a region where the indentation was different from all the
others (2 spaces instead of 4);
* replaced xrange with range.

Could someone, please, commit this?

Regards,
-- 
José Abílio
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