You can recreate a polygon from the lines. In the example below I did it via a convex hull, but there are other possibilities such as creating the polygon ring from the coordinates.
After that you can intersect this polygon with the original polygon.
from shapely.geometry import LineString, Polygon, MultiLineString
a = LineString([(0, 3), (5, 3)])
b = LineString([(0, 5), (5, 5)])
poly = Polygon([(1, -2), (1, 7), (4, 7), (4, -2)])
# Create a polygon from the lines
multi_line = MultiLineString([a, b])
line_poly = multi_line.convex_hull
# get the intersection
intersection = poly.intersection(line_poly)