Use standard Python for PySCF tooling

[mveril]

Summary

• expose PySCF and the scientific packages through the standard Python environment from the flake
• remove the need for a pyscf-python wrapper command
• update the ethanol reference command to use python

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features

Nix execution will be validated by Linux CI because Nix is unavailable on the Windows host.