Gimpformats

- Tidy up
- pyupgrade
- rasterImageOffset -> blendmodes
- Work on tests a bit

- Fix 2
- Tidy up

2021 - 2021/03/18

- Update Pillow >= 8.1.1 due to high severity security vulnerabilities:
- CVE-2021-27923
- CVE-2020-35654
- CVE-2020-35653
- CVE-2021-27921
- CVE-2021-27922
- CVE-2020-35655

- 8 failed, 34 passed in 6.76 seconds ...
- Probably needs a re-write, but I don't have time.

- A bit of typing

- Update gpl reader to remove comments
- More work on saving xcf. Still doesn't work.

- Fix fatal read xcf bug (upstream)

- `BinaryIO` is now an external dependency `binaryiotools` as it provides many
generalized methods that can be used to read a whole range of binary streams