Installation issues when installing from setup.py on python 3.8

[falkben]

Following up after my conv. w/ @hover2pi and @jaymedina and related to #387

When installing exoctk into a fresh conda environment from setup.py I noticed a few minor issues, hopefully can be fixed up by: #464

Here are the two issues I noticed:

• setup.py does not specify a version of astropy, so it installs the latest. However, I've found that the jwst_gtvt package pinned in the setup.py file conflicts with newer versions of astropy. The last working version of astropy w/ jwst_gtvt is 4.0.4. Solution might be to update jwst_gtvt (no idea if newer versions work with newer versions of astropy) or pinning astropy to <4.1. Ideally jwst_gtvt would indicate which version of astropy it worked with so then the dependency resolver would take care of the rest...
• jwst_gtvt doesn't get installed automatically unless it's put into the install_requires section of the setup.py. This I believe was a change to pip in version 19.0: https://pip.pypa.io/en/stable/news/#id449 you can add it to the requires section directly now:
  'jwst_gtvt @ git+https://github.com/spacetelescope/jwst_gtvt.git@cd6bc76f66f478eafbcc71834d3e735c73e03ed5'

Really happy I don't have to create the data directories or env. variables anymore, thanks!

[bourque]

@falkben With the merge of #469 there is now a python 3.8 environment that uses astropy<4.1. I also adopted your fix to remove the need for dependency_links, thanks for that suggestion!

The changes are now in the develop branch, and will be part of master upon our next release (end of march).

Let me know if you run into any further issues!