fixing TrytesCompatible so that its no longer generic.

[bschnurr]

Description of change

Fixes an error found by using the Pylance LS in basic typechecking mode.

I'm assuming the original code was meant to be "str or bytes". Most the docstrings mention Byte string or bytearray.
Using a constrained TypeVar like AnyStr in a Union is usually a mistake.

TrytesCompatible = Union[AnyStr, bytearray, 'TryteString'] defines a generic type alias, which can be legitimate. To use it properly, you'd need to provide a type argument that is compatible with the AnyStr type variable.

TrytesCompatible[str] or TrytesCompatible[bytes] or TrytesCompatible[AnyStr] would all be allowed.

Type of change

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• Bug fix (a non-breaking change which fixes an issue)

How the change has been tested

Ran all tests
python setup.py test

Change checklist

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• My code follows the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (docs/ directory and/or docstrings in source code)
• I have followed PEP-8 Style Guide in my code.
• New and existing unit tests pass locally with my changes