Expose signal strength info

[dbr]

I was looking to write a simple Prometheus metrics exporter to monitor signal strength over time - so I've exposed these attributes as info.signal.bars etc:

Signal(bars=5, ecio=0, rscp=0, rsrq=-9, rssi=-57, sinr=0)

Also pinned aiohttp to 3.x as there is an alpha release of 4.0.0 which was causing some warnings, and presumably might need code changes when it's fully released