Brand-new macbook pro - installed python3 using Homebrew, then followed the instructions here verbatim
except that I specified python=3.8 instead of python=3.7 when creating the environment.
obspy-runtests produces 86 errors, most having to do with matplotlib, but some also with UTCdatetime. The report is here.
Attempts to install by pinning to earlier versions of matplotlib have been unsuccessful. Any suggestions would be most welcome!