Datasources - Small refactoring

[PrimozGodec]

Issue

• While considering Fix Yahoo Finance Import #257, I found that the code can be refactored.
• Aggregation tests are failing since Scipy changed the behaviour of stats functions.

Description of changes

• Refactoring the code of the Datasources module.
• Fix weighted_mode function: Before stats.mode had keepdims=True like behaviour by default, which changed in 1.11. Since it is False, now the result is a one-dimensional vector, so I removed the part of the code that extracts the first column. I also limited the version of Scipy to >=1.9 since it is the first version offering the keepdims parameter.

Includes

• Code changes
• Tests
• Documentation