Add support for version string corner cases.

[weaverba137]

This PR allows a wide variety of valid version strings to be parsed by desiutil.setup.get_version().

While developing this, it was discovered that desiutil.dust.ext_fitzpatrick() requires scipy, but this requirement had never been explicitly listed. It appears that previously, one or more of astropy, healpy or matplotlib would bring in a dependency on scipy, but more recent versions do not have that dependency, so scipy has to be installed explicitly.

[sbailey]

Support for corner case version strings + tests looks good.

Commentary on the optional scipy dependency:
If pytest is run in an environment without scipy installed, just one of the dust tests fail with an ImportError, and all of the others run and pass, which seems good. I had originally suggested that we intercept this case and explicitly skip the test that needed scipy, but a single failing test for an obvious reason (ModuleNotFoundError: No module named 'scipy') is clear and doesn't prevent other tests from being useful, so I like this too.