Bump version to 0.4.20

[djc]

I think it's time to unleash this thing!

Added #731, #747, #744 and #746 to the release notes.

[esheppa]

Sounds good to me! All the remaining (recent) issues are API-incompatible so it is great to get this out first

[tdkt]

Is there a typo in the changelog about minimal compatible rust version? It mentions both 1.32 and 1.52.

[djc]

The minimum supported Rust version for 0.4.20 is 1.32.0, which is intentionally still quite conservative. If you are using chrono 0.4 with a Rust version older than 1.52, we'd like to hear from you since we'd like to further modernize the code base to ease maintenance.

What part of this is unclear?

(FWIW I've since heard from people who want to use 1.48 so we might stick with that instead of 1.52.)

[tdkt]

Sorry, my bad after re-reading it attentively it makes sense.