icons.zip and std.zip are changed after the test suite is run

While developing, one often runs the test suite locally before pushing their changes. After running the test suites, these two files are reported to be changed by git:

```
Changes not staged for commit:
(use "git add ..." to update what will be committed)
(use "git restore ..." to discard changes in working directory)
modified: pyface/image/library/icons.zip
modified: pyface/image/library/std.zip
```

Then what often happens is that developers did not notice that and commit these changes as part of their PR. However these changes are orthogonal and had to be reverted (or otherwise every PR will be creating merge conflicting with other PRs). This has bitten at least 3 developers as far as I can tell.

See this comment: https://github.com/enthought/pyface/pull/769#issuecomment-710082605