v0.4.21

0.4.21 is a bugfix release that mainly fixes one regression from 0.4.20:

• Fall back to UTC in case no timezone is found. Unfortunately this is a regression from the changes we made in 0.4.20 where we now parse the timezone database ourselves. Before 0.4.20, TimeZone::now() fell back to UTC in the case it could not find the current timezone, but the new implementation panicked in that case.
• Correctly detect timezone on Android (also #756). Android does have the timezone database installed, but it's in a different path, and it does not use /etc/localtime to keep track of the current timezone. Instead we now use the iana-time-zone crate as a dependency, since it already has quite a bit of logic for finding the current timezone on a host of platforms.

Additionally, there is a documentation fix that reverts an incorrect guarantee:

• Document that %Y can have a negative value, both in formatting and in parsing (#760, thanks to @alex)