Investigate moving CaChannel -> pyepics

Following a discussion with @FreddieAkeroyd ...

We currently use `CaChannel` python bindings. These are not particularly actively maintained nowadays, and we are gradually moving into a situation where we have a de-facto hard fork of it anyway.

The EPICS community at large seems to have _mostly_ settled on `pyepics` for `sync` epics CA bindings (for `async` bindings, we're already using `aioca` in e.g. bluesky). It can work with `epicscorelibs` using `epicscorelibs.path.pyepics` and/or explicitly setting env variables.

Consider whether we could/should move to `pyepics` as opposed to `cachannel` as our primary underlying synchronous CA bindings for user code.