Fixes#186
* Tread the permissions (the /P entry) as unsigned long, fix#186
* handle negative values for p
* Extract function for resolving an twos-complement
* Add test for issue #352
* Add line to CHANGELOG.md
* Only ints can be converted to a uint using two's-complement method
* Standardize import style; multiple imports from same module on one line
Co-authored-by: Pieter Marsman <pietermarsman@gmail.com>
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