Fix uv sync / uv venv with pytest

uv sync
source .venv/bin/activate
cd test
python -m pytest .

- Update default bundled capnproto to 1.4.0
- Cleanup pyproject.toml
- Remove Pipfile and requirements.txt

README.md

@@ -77,7 +77,7 @@ While not directly supported by pycapnp, a tool has been created to help generat
 
 ## Python Versions
 
-Python 3.8+ is supported.
+Python 3.9+ is supported.
 
 
 ## Development