InOutFlow broken with python >=3.11

[jc-harrison]

Describe the bug
flowmachine.features.location.flows.InOutFlow has a direction parameter to specify whether the query is an inflow or an outflow. We use the following enum to ensure the direction is either "to" or "from":

class Direction(str, Enum):
    OUTFLOW = "from"
    INFLOW = "to"

The str mixin ensured that Direction objects could be used as strings, e.g. f"pcod_{self.direction}" formats to either "pcod_from" or "pcod_to".

But python 3.11 changes this behaviour (see https://blog.pecar.me/python-enum for a description of the change), so that f"pcod_{self.direction}" formats to "pcod_Direction.INFLOW" or "pcod_Direction.OUTFLOW", which breaks the in/outflow SQL query.

Python 3.11 introduces a StrEnum class that can be used instead. So either we can make the definition of Direction conditional on the python version, or we can explicitly use direction.value instead of direction for string formatting.

I suspect there are other places we use the class MyEnum(str, Enum) hack, which may also be broken.

[greenape]

Ouch, there are a bunch. Could replace with strenum and make 3.11 the min version?

[jc-harrison]

Ouch, there are a bunch. Could replace with strenum and make 3.11 the min version?

Probably sensible. It's about time we bump the python version in the docker image anyway.