Why don't we test against GAP 4.10 in the CI? (Should we be requiring GAP 4.11?)

[wilfwilson]

I'm sure I could work out the answer, but I don't have the time right now so I thought I'd made an issue while it's on my mind.

I noticed that Digraphs currently requires only GAP 4.10, however the oldest version of GAP that we test against is the latest version of GAP 4.11.

I'm guessing that there's a fiddly technical reason why we don't test against GAP 4.10; that's possibly good enough reason for us not to support GAP 4.10 as well. Let's look into it. (Btw, GAP 4.11.0 was released in February 2020; GAP 4.10.0 was released in November 2018).

[wilfwilson]

Am working on this in #846.