Support sqlalchemy 2.0

[txemi]

I have seen sqlalchemy 2.0 mentioned in other issues related to support sqlalchemy 1.4, but none had explicit mention for 2.0 in tittle, so I created this one.

I my case I had to go back to 1.4 and change previous code in an application that was already working with sqlalchemy 2.0 in order to add functionality to make it able to export accounting data to gnucash format, so I would thank implementing sqlalchemy 2.0 in piecash.

I decided to go back to 1.4 after going crazy trying to adjust library version dependencies in my project to keep up to date with recent libraries or finding simple workaround for it until I realised that It was too complex and decided to make all code in project sqlalchemy 1.4 compatible.

Thanks,

[sdementen]

hello @txemi, see #231