build(deps): bump structopt from 0.3.15 to 0.3.21

[dependabot]

Bumps structopt from 0.3.15 to 0.3.21.

Changelog

Sourced from structopt's changelog.

v0.3.21 (2020-11-30)

• Fixed another breakage when the struct is placed inside a macro_rules! macro.

v0.3.20 (2020-10-12)

• Fixed a breakage when the struct is placed inside a macro_rules! macro.

v0.3.19 (2020-10-08)

• Added StructOpt::from_args_safe as a shortcut for StructOpt::from_iter_safe(std::env::args_os()).
• Some links in documentation have been corrected.

v0.3.18 (2020-09-23)

• Unsafe code has been forbidden. This makes cargo geiger list structopt as "safe". Maybe it will help somebody trying to locate a bug in their dependency tree.

v0.3.17 (2020-08-25)

• Fixed a breakage with resent rustc versions due to quote_spanned misuse.

v0.3.16 (2020-08-05)

• Added the new example.
• Allow #[structopt(flatten)] fields to have doc comments. The comments are ignored.
• The paw crate is now being reexported when paw feature is enabled, see [#407](https://github.com/TeXitoi/structopt/issues/407).

Commits

• 8bda3b6 v0.3.21
• df5c7c9 Adjust .stderr files
• 399265f cargo fmt
• 583eb82 Fix #447
• 6fa8d68 v0.3.20
• 4c17b61 cargo fmt & clippy
• 0852b91 Fix #439
• 1cc5b05 v0.3.19
• 1cd8405 cargo fmt
• 98d5d80 If you see me, master is not as protected as we hoped
• Additional commits viewable in compare view

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[dependabot]

Dependabot tried to add @dconnolly as a reviewer to this PR, but received the following error from GitHub:

POST https://api.github.com/repos/str4d/zebra/pulls/7/requested_reviewers: 422 - Reviews may only be requested from collaborators. One or more of the users or teams you specified is not a collaborator of the str4d/zebra repository. // See: https://docs.github.com/rest/reference/pulls#request-reviewers-for-a-pull-request

[dependabot]

Superseded by #31.