Support Python 3.13 and drop Python 3.9

[tyralla]

I checked our dependencies, and we still have to wait for suitable NetCDF4 binaries (but they are already in preparation, NetCDF4 issue 1370). Hence, the following to-do list is only preliminary:

• Use the | operator instead of typing.Union (see PEP 604).
• Clean up ugly multi-line with statements like this one by using parentheses (see Python issue 56991)

[tyralla]

Proper NetCDF4 wheels are now available. However, we must wait for Python 3.13.1 because of the bug reported here.

[tyralla]

I hoped that PEP 667 would allow us to manipulate the (optimised) namespaces of functions, too, so that patterns like this became possible:

from hydpy.models.hland_96 import *

simulationstep("1h")
parameterstep("1d")

def add_rconcmodel(main):
    with main.add_rconcmodel_v1("rconc_uh"):
        uh("triangle", tb=0.36728)

add_rconcmodel(model)

The error under Python 3.12 is: NameError: name 'uh' is not defined.

The (current) error under Python 3.13.0 is: AttributeError: 'FrameLocalsProxy' object has no attribute 'pop'.

The latter message will likely change under Python 3.13.1 due to PR 125616.

However, adding and removing names from optimised scopes will still not be supported, so everything will stay as is from the user's perspective. (We might need to adapt HydPy's source code, but I expect at most minor changes.).

[tyralla]

I completed the above two tasks and a little more refurbishing (with the help of PyUpdater) and pushed everything into the branch feature/python_3.13.

That's it for now. I will wait for Python 3.13.1, which can apparently be expected for December.