Update strum_macros requirement from 0.20 to 0.21

Updates the requirements on [strum_macros](https://github.com/Peternator7/strum) to permit the latest version.
- [Release notes](https://github.com/Peternator7/strum/releases)
- [Changelog](https://github.com/Peternator7/strum/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Peternator7/strum/commits)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.toml

@@ -54,7 +54,7 @@ bitflags = "1.2"
 libc = "0.2"
 enum-primitive-derive = "^0.1"
 num-traits = "^0.2"
-strum_macros = "0.20"
+strum_macros = "0.21"
 #failure = "0.1"
 
 [build-dependencies]