Update PeakRDL plugin to use base class

[amykyta3]

As you might have seen, I reworked how PeakRDL handles plugins slightly.
Starting in v0.7.0, all importer & exporter plugin descriptor classes now have to be derived off of a base class.
This is something I initially hesitated to do, but after a while found it cumbersome to not have proper class inheritance here.
Unfortunately this kinda breaks compatibility, but hey this is all beta stuff so far anyways and I was bound to change my mind on these things 😉

This PR addresses the compatibility change.

[MarekPikula]

Thank you for the change 😃

Besides, would you be interested in pulling this plugin to the SystemRDL namespace? It would be pretty nice to have it properly "upstreamed".

[amykyta3]

Yes always interested in more useful tools to be added!
A few questions:

• Does your markdown plugin work on py3.6 and py3.7? Sure these are pretty old versions, however it is not uncommon for people to be stuck with an older python version in their workplace. For that reason, I strive to keep compatibility back to py3.6 for all the other PeakRDL tools.
• Would you be willing to move the repository to the SystemRDL organization (and join the group)? You would retain all existing permissions & ownership as before.

[MarekPikula]

1. Python 3.7 is supported (though not from 3.7.0, but 3.7.2). I think it was because some development dependency required version ≥3.7.2, but it should be easy to change down to 3.6. As far as I recall, I didn't use any advanced features, so it's not an issue as well. I'll check today.
2. Sure, that would be great.

[MarekPikula]

Actually, now I checked, and many development dependencies (including pytest) require Python ≥3.7.0. So I could go as low as 3.7.0, but I'm afraid that no lower than that, as I would have no means to execute reliable tests in CI (here, I'm using pytest). Would that work for you? Anyway, which LTS distro uses Python 3.6?

[amykyta3]

I sent you an invite. Once added you should be able to move the repository.

I'm using pytest all the way down to 3.5 without issues.
Ubuntu 18.04 = py3.6
Ubuntu 16.04 = py3.5
Both distributions are under extended maintenance for still a few more years. Similar for CentOS as well.

I wouldn't worry about the version stuff too much. Should be pretty straightforward to back-fill support.

[MarekPikula]

I just noticed that you sent me the invite. It must have missed it in the inbox. Could you resend it?