PyPy support with test pipeline

[evnchn]

Motivation

Further develop on #5161. For PyPy to be first-class (or second-class) citizen of NiceGUI it has to, at the very least, run our test suite

Implementation

• Add PyPy into the pipeline.
• Expand the timeout to 120 minutes for now
• Drop LangChain, OpenCV, numpy (and other things which use numpy) from test
• Drop reference counter related test
• Loosen timing for event tests

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests are not necessary.
• Documentation is not necessary.

[evnchn]

Missing pydantic core. Huh. But pydantic was installed, no?

https://github.com/evnchn/nicegui/actions/runs/17921019299

[evnchn]

pydantic-core is a rust component. Until we figure out why it fails in the pipeline (it doesn't fail installing on my machine) we can use Pydantic V1 for now: https://github.com/fastapi/fastapi/blob/master/pyproject.toml#L31