auth: support both old and new python-pam API python-pam 1.8.* provides a new API replacing the previously available authenticate() function. The latest unreleased version brings it back, but until it's available, use a custom shim instead.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.