Drop support for python 3.9

[hboisgon]

Kind of request

Removing existing functionality

Enhancement Description

We now have the issue that python 3.9 is not tested or supported because of
pipe typing nodata_value: float | int = -9999 in demand.py#L411 which is only allowed for python >=3.10.

3.9 support is being dropped for different packages but for hydromt-wflow I think it would be better to drop it once we migrate to hydromt v1 #275 and possibly wflow v1 #319
So the above issue will be fixed for now.

Use case

No response

Additional Context

No response