build(deps): update proc-macro2 requirement from =1.0.79 to =1.0.81

[dependabot[bot]]

Updates the requirements on proc-macro2 to permit the latest version.

Release notes

Sourced from proc-macro2's releases.

1.0.81

• Documentation improvements

Commits

• fd2c998 Release 1.0.81
• f965e17 Merge pull request #453 from dtolnay/delimiternone
• 69fd060 Sync documentation of Delimiter::None
• da51f8d Release 1.0.80
• 392fa6c Ignore needless_pass_by_value pedantic clippy lint in test
• 1ff5c1f Ignore needless_raw_string_hashes pedantic clippy lint in test
• 0c9449e Import some Literal tests from libproc_macro
• 28c87ad Add tests of negative literal construction
• c61f5db Add some tests of cstr raw literals
• d50dcd9 Merge pull request #450 from dtolnay/cstr
• Additional commits viewable in compare view

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[Rahix]

Ref #537.

It's interesting that CI passes for this PR. There must be some caching happening that we need to be careful about. Ideally, CI would catch problems like #537...

[dependabot[bot]]

Superseded by #548.