Shapely has a parallel_offset method (https://shapely.readthedocs.io/en/latest/manual.html#object.parallel_offset).

This seems to be based on GEOS' GEOSOffsetCurve (and in older GEOS versions on GEOSSingleSidedBuffer, see https://github.com/libgeos/geos/blob/34b8d63e72353166d149db71bf9c45faa0148aaa/capi/geos_c.h.in#L466-L482)

PostGIS also calls this ST_OffsetCurve (https://postgis.net/docs/ST_OffsetCurve.html)