Calling mac M1 users to try pip-installable httpstan branch

@ariddell and I discussed making httpstan pip installable from source. This should allow M1 users (and more generally any architecture for which pre-compiled wheels do not exist) to use httpstan (which is required for the most recent version of pystan).

There is a branch which should achieve this goal, and it would be interesting to see whether it works for others or just for me. If you have an M1 mac (or another architecture), could you please run the following from the console and report back as to whether it works?

# Uninstall httpstan in case it's already installed.
pip uninstall -y httpstan pystan
# Install httpstan from source (this will take a while).
pip install -v git+
# Install pystan.
pip install 'pystan>=3'
# Try a dummy program (this should report build and sampling progress).
python -c 'import stan;"parameters { real x; } model { x ~ normal(0, 1); }").sample()'

Hi just to comment that this worked for me, I used it in a new virtual env