Replace `-` with `:` in slices indexes.

[python-processing-unit]

According to the specification section 2.2:

The character - MUST be interpreted only as the leading sign of a numeric literal or as the dash used by slice syntax lo-hi. Any unsupported use of - MUST raise a syntax error.

and the specification section 4.4.2:

Slice indexing MUST be supported in any index position using a range of the form lo-hi. The selected slice MUST be inclusive of both endpoints.

Use colon : instead of dash - between the beginning and ending of slices.

Update the implementation, specification, tests, and stdlib.