Use pytest.importorskip to skip on a missing import dependency

[hugovk]

For #4193.

Changes proposed in this pull request:

• First commit: Use pytest.importorskip to skip on a missing import dependency
  • https://docs.pytest.org/en/latest/skipping.html#skipping-on-a-missing-import-dependency
• Second commit: Also convert the touched files to use pytest

[radarhere]

Just so you're aware, this is an alternative to #4434

[hugovk]

This actually covers something different to #4434: that skips tests when features are not available, this skips tests when imports are not possible :)

[hugovk]

Ah, there was a bit of overlap for a bit of the WebP checking. Will update this to keep those as feature checking.