8.0.13 introduced a breaking change because the namespace changed requiring updating code and a recompile

[ramonsmits]

using NHibernate.Extensions.NpgSql;

SetProperty(Environment.ConnectionString, "xxx");
SetProperty(Environment.ConnectionDriver, typeof(NpgSqlDriver).AssemblyQualifiedName);
SetProperty(Environment.Dialect, typeof(NpgSqlDialect).AssemblyQualifiedName);