Add matplotlib dependency to setup requirements

[sphuber]

It cannot be in the 'docs' optional group, because then the
pre-commit hook won't pick it up to allow it in the requirements
text file for RTD, which then won't install it

[ltalirz]

@sphuber Are you aware of this: #1144
(perhaps develop was not merged into workflows since?)

[giovannipizzi]

Thanks Leo, I was looking for #1144 but couldn't find it (there are a few more PR about docs ;-) )
Maybe indeed it's enough to merge develop into workflows?

[sphuber]

Thanks Leo, I wasn't aware of that issue. Giovanni notified me of the failing workflows builds and asked me to add matplotlib. So am I to understand that this was already fixed in develop? If so, I will close this and just merge develop into workflows and open a PR for that. I was planning on merging develop anyway

[ltalirz]

Yes, just merge develop and see whether the problem goes away