I feel your pain Python x Ansible x Trellis versions can be a nightmare to get right!
These are the steps I’ve been using to get them all working smoothly together on macOS Catalina
# Install Python 3.7 with pyenv using homebrew
brew install pyenv
pyenv install 3.7.6
pyenv global 3.7.6
pyenv rehash
pip install --upgrade ansible==2.7.15
As far as I know, the latest versions of Trellis need ansible version 2.8, so change that last line to pip install --upgrade ansible==2.8.11 if you’re working on a fairly fresh clone of Trellis.
One of the guys on our team started using Ansible Version Manager (AVM) which seems to be working pretty well - hope it helps!