attempt to wrap the attrs mypy plugin

[wpbonelli]

Very rough draft. Stealing the general approach from the attrs plugin. This can be successfully imported and used from our flopy4 prototype, but it still doesn't properly set up the __init__ method signature, for some reason that til now escapes me.

E.g. when run on the quickstart

docs/examples/quickstart.py:16: error: Unexpected keyword argument "name" for "Simulation"  [call-arg]
docs/examples/quickstart.py:17: error: Unexpected keyword argument "parent" for "Ims"  [call-arg]
docs/examples/quickstart.py:19: error: Unexpected keyword argument "parent" for "Gwf"  [call-arg]
docs/examples/quickstart.py:19: error: Unexpected keyword argument "name" for "Gwf"  [call-arg]
docs/examples/quickstart.py:20: error: Unexpected keyword argument "parent" for "Npf"  [call-arg]
docs/examples/quickstart.py:21: error: Unexpected keyword argument "parent" for "Chd"  [call-arg]

There are more errors because some __init__ arguments passed don't match their expected types, because we're using converters. That can be remedied pretty easily, I am just focusing on getting rid of the errors above first.