Fix invalid escape sequence messages

[fredsonnenwald]

When running

from omymodels.models.pydantic import types

under Python 3.12 I get

<unknown>:1: SyntaxWarning: invalid escape sequence '\#'
<unknown>:1: SyntaxWarning: invalid escape sequence '\-'

Based on erikrose/parsimonious#246 this seems to be due to Python getting more aggressive about escape sequences. It also looks like there's a fairly straightforward fix that involves marking the string as a raw string, which I've done in this patch. After making that change the invalid escape sequence messages go away.