BUG: better handle optional dependencies for testing

[bsipocz]

There are a couple of optional dependencies that are seemed to be pulled in for all the test runs and thus we don't notice if a module using it without padding for them not being available. Even if we decide that there are required dependencies, the testing infrastructure needs to be fixed.

E.g:

• pillow is used in hips2fits while it could be not available.
• botocore, as reported in mast import crashes when botocore optional dependency is not installed #3546