Migrate to pydantic v2; drop unmaintained streamlit_pydantic

[thorwhalen]

Closes #9.

Why

front has moved to pydantic v2 (i2mint/front#30, merged). streamlitfront consumes front.py2pydantic and was the next package keeping the ecosystem (extrude, anything composing front elements) pinned to pydantic v1.

The blocker: streamlit_pydantic

streamlit_pydantic's last release — 0.6.0 (2022) — imports pydantic.BaseSettings, removed in pydantic v2. There is no v2-compatible release on PyPI; the package is effectively abandoned.

Rather than pin the whole ecosystem to pydantic v1 for the sake of one
unmaintained dependency, the small subset of streamlit_pydantic that
streamlitfront actually used is reimplemented natively against pydantic v2
and streamlit's own widgets.

What changed

streamlitfront/pydantic_widgets.py (new, ~115 lines)

A drop-in replacement for import streamlit_pydantic as sp:

function	behaviour
pydantic_input(key, model)	one widget per field, returns a validated model instance (or None on validation error)
pydantic_form(key, model)	same, wrapped in st.form + submit button; returns the instance on submit, else None
pydantic_output(instance)	renders the instance as JSON

Widget type is chosen per field from model.model_fields (bool → checkbox,
int/float → number_input, everything else → text_input; Optional[X] is
unwrapped to X).

streamlitfront/page_funcs.py

SimplePageFuncPydanticWrite / SimplePageFuncPydanticWithOutput now use
streamlitfront.pydantic_widgets. Result handling uses
instance.model_dump() rather than dict(instance).

streamlitfront/tests/test_pydantic_widgets.py (new)

• Unit tests for the pure helper (_unwrap_optional).
• Integration tests via streamlit.testing.v1.AppTest: render the
  widgets in a headless Streamlit run, edit them, and confirm a validated
  model instance is produced; confirm pydantic_form returns None until
  submitted.

streamlitfront/tests/dummy_app.py

Same import swap; added the missing module docstring.

setup.cfg

• Drop streamlit_pydantic.
• Add pydantic>=2 explicitly (it was imported at module-load time but
  never declared).

Migration guide for downstream code

If you imported streamlit_pydantic via streamlitfront, switch to
from streamlitfront import pydantic_widgets as sp — the three functions
above keep the same names. Note pydantic_input / pydantic_form now
return a pydantic model instance (not a dict); use .model_dump() if
you need a plain dict.

General pydantic v1→v2 changes (see front#30) also apply:

v1	v2
model.__fields__	model.model_fields
instance.dict()	instance.model_dump()
model.schema_json()	json.dumps(model.model_json_schema())

Verified

$ python -m pytest streamlitfront/tests/test_base.py \
    streamlitfront/tests/test_pydantic_widgets.py
======================== 5 passed, 3 warnings in 0.17s ========================

All imports resolve with no streamlit_pydantic installed, under
pydantic 2.12.

Follow-up

extrude can now drop its temporary CI workarounds (Python 3.12 + Windows
skip) once a new streamlitfront is on PyPI.