Fix some testgen issues related to computed constants

[jtraglia]

This PR fixes some small side effects of this change:

• Simplify build system for computed constants #4152

This the error log here:

• https://github.com/ethereum/consensus-specs/actions/runs/13755377250/job/38461868261

Notably:

  File "/opt/actions-runner/_work/consensus-specs/consensus-specs/consensus-specs/venv/lib/python3.12/site-packages/eth2spec/eip7732/mainnet.py", line 348, in <module>
    PAYLOAD_TIMELY_THRESHOLD = PTC_SIZE / 2
                               ~~~~~~~~~^~~
  File "/opt/actions-runner/_work/consensus-specs/consensus-specs/consensus-specs/venv/lib/python3.12/site-packages/remerkleable/basic.py", line 120, in __truediv__
    raise OperationNotSupported(f"non-integer division '{self} / {other}' "
remerkleable.basic.OperationNotSupported: non-integer division '512 / 2' is not valid for uint64 left hand type

and

  File "/opt/actions-runner/_work/consensus-specs/consensus-specs/consensus-specs/venv/lib/python3.12/site-packages/ruamel/yaml/representer.py", line 354, in represent_undefined
    raise RepresenterError(_F('cannot represent an object: {data!s}', data=data))
ruamel.yaml.representer.RepresenterError: cannot represent an object: 127