Add numbers as Decimal objects

[jtherrmann]

Fixes #7349

TODO:

• I've only updated the DynamoType.__add__ method, but there are other places where floating-point arithmetic is performed that should probably be updated as well, e.g. DynamoType.__sub__.
• Add test cases for these changes?
• Fix failing tests

On my machine, make test fails with:

moto/dynamodb/models/dynamo_type.py:109:48: error: Unsupported left operand type for + ("object")  [operator]
Found 1 error in 1 file (checked 900 source files)
make: *** [Makefile:30: lint] Error 1

[bblommers]

Hi @jtherrmann, thanks for getting this started. Do you want to tackle the TODO's yourself, or are you OK if someone else picks this up?

[jtherrmann]

@bblommers I'd love it if someone else could finish it off!

[bblommers]

@jtherrmann No problem! I wasn't able to push to your branch, so I've created #7365 that contains your commit + some tests.

Regarding your TODO's:

but there are other places where floating-point arithmetic is performed that should probably be updated as well

Only ADD is supported/required for numerical values actually, so I think this is all that is necessary. From the docs:

If the existing attribute is a number, and if Value is also a number, then Value is mathematically added to the existing attribute. If Value is a negative number, then it is subtracted from the existing attribute.

The failing test was mypy complaining, because of a known issue where it can't find a suitable parent-type: python/mypy#3816.
As far as I can tell, the only way around this is to manually decide on a suitable type (Union[Decimal, int])