According to the [changelog](https://matplotlib.org/stable/users/prev_whats_new/whats_new_3.6.0.html#webp-is-now-a-supported-output-format), it was added in 3.6.0. `pip install matplotlib -U` in the miloPYP environment installed 3.7.5 which works. 
According to the changelog, it was added in 3.6.0.
pip install matplotlib -Uin the miloPYP environment installed 3.7.5 which works.