Bump chrono from 0.4.20 to 0.4.21

[dependabot]

Bumps chrono from 0.4.20 to 0.4.21.

Release notes

Sourced from chrono's releases.

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)

Changelog

Sourced from chrono's changelog.

ChangeLog for Chrono

This documents all notable changes to Chrono.

Chrono obeys the principle of Semantic Versioning, with one caveat: we may move previously-existing code behind a feature gate and put it behind a new feature. This new feature will always be placed in the previously-default feature, which you can use to prevent breakage if you use no-default-features.

There were/are numerous minor versions before 1.0 due to the language changes. Versions with only mechanical changes will be omitted from the following list.

Commits

• 2d22e57 Bump version number to 0.4.21
• 557bcd5 handle missing /etc/localtime on some unix platforms (#756)
• 5edf4d0 fixes #758 -- correctly document that %Y parses negative years (#760)
• a185d3b Remove unnecessary try_opt!() macro
• See full diff in compare view

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