Replace underscored circuit library functions

[sjdilkes]

No description provided.

[cqc-melf]

@sjdilkes the scheduled run failed with:

ImportError while loading conftest '/home/runner/work/pytket-ionq/pytket-ionq/tests/conftest.py'.
conftest.py:18: in <module>
    from pytket.extensions.ionq import IonQBackend
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/pytket/extensions/ionq/__init__.py:20: in <module>
    from .backends import IonQBackend
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/pytket/extensions/ionq/backends/__init__.py:18: in <module>
    from .ionq import IonQBackend
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/pytket/extensions/ionq/backends/ionq.py:40: in <module>
    from pytket.circuit_library import TK1_to_RzRx  # type: ignore
E   ModuleNotFoundError: No module named 'pytket.circuit_library'

See https://github.com/CQCL/pytket-ionq/actions/runs/6859045945

Is that related to this PR?