Siobrultech-protocols

* Drops support for Python 3.7
* Improved typing information
* Added helper methods on `Packet` to compute deltas
* `delta_seconds`
* `delta_pulse_count`
* `delta_absolute_watt_seconds`
* `delta_polarized_watt_seconds`
* Support bi-directional communication with a GEM. Currently supports only one command to get the serial number of the device. ([docs](https://github.com/sdwilsh/siobrultech-protocols/blob/v0.3.0/README.md#receiving-data-packets-and-sending-api-commands))

Corrects incorrect typing of `Packet.polarized_watt_seconds` from `Optional[int]` to `Optional[List[int]]`.

This fixes the minimum supported Python version from 3.8 to 3.7.

Initial version that currently only supports the [GreenEye Monitor](https://www.brultech.com/greeneye/) based off of [jkeljo/greeneye-monitor](https://github.com/jkeljo/greeneye-monitor).