Adding more pytest-mpl maintainers

[Cadair]

@ConorMacBride has been making lots of valuable contributions and @bjlittle expressed interest in helping out (#150 (comment)) so could you add them @dopplershift or @tacaswell ?

[Cadair]

Also you good with this @astrofrog ?

[astrofrog]

Yep!

[tacaswell]

Invitations are out, but I think both @Cadair and @astrofrog should have the power to invite collaborators to pytest-mpl (which is slightly different than adding to the org).

[bjlittle]

@Cadair @astrofrog @dopplershift and @tacaswell Thanks so much 🥳🎉🍻