Enable SLF001 ruff check

[coretl]

As per DiamondLightSource/python-copier-template#154

[stan-dot]

SLF001 is about private class method/property access

[coretl]

Waiting for #383

[coretl]

@abbiemery I merged this into the copier template main, please could you try doing a copier update --vcs-ref=main and check this works?

[abbiemery]

I did the changes to ophyd-async that are JUST the SLF ones, as the other ones on the copier template bring in a lot of errors to fix. I suggest we merge the SLF changes now, and make issues to adopt the other ones.

#483