python - Add CRS to a shapefile in Geopandas?

I am trying to add CRS to a shapefile that I am exporting:

shp1= gpd.read_file(i[1],encoding='iso8859_7',crs_wkt='2100')
shp1.to_file(out,encoding='iso8859_7',crs_wkt='2100')

When I import it to a GIS later, it doesn't have the CRS that says in crs_wkt.

I tried to do this:

shp1=shp1.to_crs(epsg=2100)

but says:

RuntimeError: b'no arguments in initialization list'

If check for the crs it has

shp1.crs

it says:

{}