Modernize some bits in the introduction

[HalfWhitt]

Hi there! I just happened upon Traits, read the intro, and noticed some spots in it that could stand to be updated.

• Pathlib's been around since 3.4, and is now so ubiquitously used and supported that I'd say it no longer requires a parenthetical for "advanced users".
• F-strings have been supported since 3.6, and it looks like Traits now requires >= 3.8.
• The standard library gained a cached_property in 3.8, so it's probably worth pointing out the distinction.

I don't see a contributing guide, so my apologies if I'm missing any procedures.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in stub files