Remove `timezonefinder` dependency

[llucax]

What's needed?

This dependency alone is 50MB and also depends on multiple big libraries, like numpy, which in itself is another 16MB.

All of this is for the remote possibility that someone would want to know the timezone of a particular microgrid, which seems a bit bloated.

On top of that, because of the size bloat, it seems like the package doesn't provide pre-built binaries (at least for ARM) in PiPy anymore.

Proposed solution

1. Remove the timezonefinder dependency
2. Stop attempting to find out the timezone of a microgrid when the timezone doesn't come in the protobuf message and just allow having a None timezone
3. Document the change of behaviour and dependencies in the release notes. Recommend users installing timezonefinder themselves if they need to get a timezone from the coordinates (with a small example). Clarify this change is shipped in a patch release even if there is a breaking change because this feature have no known users.