inline script deps

[zzstoatzz]

makes all flows/ self contained with inline script deps so all examples can be run:

via cli if cloned

uv run flows/hello_world.py

via url

uv run https://github.com/PrefectHQ/examples/blob/main/flows/hello_world.py

and then you don't need to install unrelated deps to try a specific example

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this pr also removes .python-version, uv.lock and the dependencies from the pyproject.toml since this is not intended to be built as a package

stacked on #1

[EmilRex]

:hell-yeah: