Ensure *--update* includes added extensions in setup.cfg

[abravalheri]

Prior to this change, new extensions added with --update were not being added to the [pyscaffold] section of setup.cfg.

[coveralls]

Pull Request Test Coverage Report for Build 5084425416343552

• 12 of 12 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 97.285%

---

Totals
Change from base Build 5013985301889024:	0.1%
Covered Lines:	1634
Relevant Lines:	1666

---

💛 - Coveralls

[FlorianWilhelm]

Oh nice! So that's a long-standing bug fixed then? Cool.

[abravalheri]

Thanks @FlorianWilhelm!
Yeah, it is one of those things that I have noticed briefly on the past, but never really paid attention to check if it was actually a bug or just something I was missing...
But working on PyScaffold this weekend I had the chance to investigate it better.