lsmtool requires older astropy version

[maaijke]

lsmtool 1.4.3 requires astropy<4.0,>=3.2, but you'll have astropy 4.3.1 which is incompatible.

[gmloose]

I recall we had some issues when astropy 4 saw first light. We probably need to see if this constraint is still necessary, because it's undesirable to set an upper limit on a version.

[darafferty]

The upper limit was removed a little while ago in #23, as indeed the constraint doesn't seem to be necessary any longer.