Fix Kraken futures symbols of alternative lengths

[roelvv]

Description of code - what bug does this fix / what feature does this add?

Kraken Futures does not handle the parsing of symbols correctly for base currencies that are not of a length of 3 chars. This causes incorrect symbols, for example:

• ALI-CEUSD-PERP, while it should be be ALICE-USD-PERP.
• OPU-SD-PERP, while it should be OP-USD-PERP

Testing

I did a diff on the symbols before and after the change and verified they are correct.

• [X] - Tested
• [X ] - Changelog updated
• [X] - Tests run and pass
• [X] - Flake8 run and all errors/warnings resolved
• [ ] - Contributors file updated (optional)