Use SPDX license expression

[gerritsangel]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

• [x] I ran cargo fmt

Uses a SPDX license expression as recommended by Cargo.

This enables e.g. parsing via the CycloneDX Rust Cargo Plugin.

[lnicola]

@bors r+

[lnicola]

Filed #107 for the CI failure.

[lnicola]

bors retry

[lnicola]

bors r+

[bors[bot]]

Build succeeded:

• wkt