Temporary pin astropy & remove python 3.6 support
This addresses problems with the C.I testing
- Temporarily force
astropy<4.3
while we wait for agwpy
release. - Remove python 3.6 support: we depend on
corner
which depends onarViz
which has dropped python 3.6 based on NEP 29. - Added the python 3.8 sampler tests
Edited by Gregory Ashton