Use `Enum` instead of string literals for `vtype`

[kjartan-at-bravos]

For instance,

import polars as pl import pyoframe as pf m = pf.Model() m.x = pf.Variable(pl.DataFrame([1,2,3]), vtype=pf.BINARY)

[staadecker]

Done.

Although you'd need to do pf.VType.BINARY. (I like to do from pyoframe import ... to make the code less verbose)