Test pip install and vignettes on PyPI release

[levon003]

After a PyPI release, test installing the latest version.

For test pypi, would be reasonable to just install:

pip install -i https://test.pypi.org/simple/ python-flexeval

For actual pypi, would be nice to do a little more:

uvx --with python-flexeval python vignettes/*.py 

Also update DEVELOPMENT.md if we do this.

We could also verify that the version the package reports is the version we think we just released:

INSTALLED_VERSION=$(uvx --with python-flexeval python -c "from importlib.metadata import version; print(version('python-flexeval'))")
# somehow check that INSTALLED_VERSION == the current tag, for example