Remove Python-3.7-specific code once scyjava stops supporting it

[gselzer]

Python 3.7 will reach end-of-life in June. With that in mind, we should bump the minimum Python version to 3.8.

When doing so, we might want to grep for any hacks we have for Python 3.7. For example, this conditional (this was the main reason I'm writing this issue, because it would be nice to get rid of this 😅)

[ctrueden]

Why do we need to ditch support for Python 3.7? To reduce the size of the CI matrix? Any other reasons to do it proactively (i.e. when it's not causing any problems), as opposed to reactively (i.e. when a problem crops up due to our continued support for 3.7)?

[gselzer]

I actually would also prefer to do it reactively, like I said in the issue I just wanted to remember to delete that conditional when we eventually do need to bump the version 😅

[ctrueden]

@gselzer Sounds good, I changed the title accordingly.