Proposal: Blender v2.7x compatibility.

[mechanic-kharkiv]

I like your add-on, it helps in my workflow. And I'm often animating in older versions of blender (a lot of machines, various specs), and my favorite is 2.76. So, I have ported the source to work in this version (blender 2.76; python 3.4; rigify 0.4). Also in-between versions were tested (2.79, 2.80, 2.92, 4.0.2).

If it's interesting, take a look over my PR #44.
It might look intimidating :-) but first few commits just change the syntax and API calls massively. They are in separate commits to easier reviewing.

By the way, while fiddling with the code, there were found some glitches, which might be hard to catch in rigify 0.5 and higher (because of bone names similarity). Of course, I've fixed as much as I could. These fixes are in the same branch. Even if it's not an option to integrate 2.7x support, those fixes could be of some use.
The most affected class is ExtractMetarig (I use it the most).