MAINT: Convert almost everything to f-strings

[HaoZeke]

With flynt. Superceedes #1198, #1197, #1196, #1245.

[mattip]

This has a merge conflict

[mattip]

Does this take into account the comments in #1245?

[HaoZeke]

Does this take into account the comments in #1245?

Yup, most of them are not relevant since extern has been removed. However, for this:

Nit: it is a bit weird to be using these old versions of python (line 39?) and numpy (dunno where that comes from). If not too much trouble could you update them? If not no big deal.

I'm not sure they should be updated here, since those are part of the asv files generated for the tests, made an issue for updating them, #1280.

[mattip]

Thanks @HaoZeke