Bump smallvec from 1.4.0 to 1.8.0 in /pymodule

Bumps [smallvec](https://github.com/servo/rust-smallvec) from 1.4.0 to 1.8.0.
- [Release notes](https://github.com/servo/rust-smallvec/releases)
- [Commits](servo/rust-smallvec@v1.4.0...v1.8.0)

---
updated-dependencies:
- dependency-name: smallvec
  dependency-type: indirect
...

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