Pass seed file tab delimiter specification correctly

[treysp]

Currently, the tab character \t isn't passed from the model specification parser to the read_csv function with usable escaping.

This specification:

kind SEED (
    path 'file.txt',
    csv_settings (
        delimiter='\t',
    ),
)

Produces this error:

Error: Falling back to the 'python' engine because the 'c' engine does not support regex separators (separators > 1 char and different from '\s+' are interpreted as regex), but this causes 'low_memory' to be ignored as it is not supported by the 'python' engine.