Work around removal of CL_MIGRATE_MEM_OBJECT_CONTENT_UNDEFINED_EXT

See merge request !40

# Conflicts:
#   .gitlab-ci.yml

Add a CI job for Python 3.7

See merge request !39

Back out Apple Conda workaround from !37

See merge request !38

CI with py36, try to fix Apple CI

See merge request !37

Fix OpenGL interop segfaults on Linux

Better error msg

See merge request !36