Solve SQL Alchemy's warnings about absence of PlanoTrabalho in session

[augusto-herrmann]

Fix #140

Solve SQL Alchemy's warnings about absence of PlanoTrabalho in session, in context of the relationship with Participante.

[augusto-herrmann]

Decorre de uma mudança de comportamento no SQL Alchemy a partir da versão 2.0.

Da documentação:

An object that’s not associated with a Session, when assigned to an attribute or collection on a parent object that is associated with a Session, will be automatically added to that same Session. However, the same operation in reverse will not have this effect; an object that’s not associated with a Session, upon which a child object that is associated with a Session is assigned, will not result in an automatic addition of that parent object to the Session. The overall subject of this behavior is known as “cascade backrefs”, and represents a change in behavior that was standardized as of SQLAlchemy 2.0.