Bump proc-macro2 from 1.0.20 to 1.0.21 (#561)
Bump proc-macro2 from 1.0.20 to 1.0.21 (#561)

Bumps [proc-macro2](https://github.com/alexcrichton/proc-macro2) from 1.0.20 to 1.0.21.
- [Release notes](https://github.com/alexcrichton/proc-macro2/releases)
- [Commits](https://github.com/alexcrichton/proc-macro2/compare/1.0.20...1.0.21)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <6a4c1c4838f800d1998274cd5234e1f65c55e90c@users.noreply.github.com>
xd009642/tarpaulin,xd009642/tarpaulin