Agree on package version restrictions

[ClaraBuettner]

Before we update the installation process of eGo, we have to agree on the package versions within eDisGo and eTraGo (e.g. for pypsa and pandas).

@MoritzSchloesser: This needs to be done together with someone from RLI. We are not sure who of you would support here, feel free to hand it over! :)

[KathiEsterl]

For now, we agree to use Python 3.11 (this still needs to be verified within eTraGo; otherwise we fall back to Python 3.10) and PyPSA 0.26.2.

Long-term goal:
Upgrade to newer Python and PyPSA versions as part of the 0.5.0 release (see corresponding milestone).