Document supported Python versions for a plugin

In a recent Slack thread a user came across an issue with pipelinewise-target-bigquery (not yet in the Hub) caused by a target's dependency not being compatible with Python ≥ 3.8.

It would help if discovery.yml and the Hub documented Python versions supported by the plugin, so Meltano can eventually warn or fail when an incompatible version is being used (meltano#2854 (closed)).

Gathering this information for plugins based on the SDK will be easier, but for the rest of the ecosystem will be harder since a lot of taps and targets don't explicitly mention supported versions in their readmes.