conf.py

# ...

# Add dummy packages to allow the documentation builds to pass on systems
# not providing all requirements.

class dummy(object):
    def __init__(self, *dummy):
        self.dummy = set(dummy)
        sys.meta_path.append(self)

    def find_module(self, module_name, package_path):
        return self if not package_path and module_name in self.dummy else None
    
    def __getattr__(self, _):
        return self
    
    def load_module(self, _):
        return self

dummy('argparse', 'lxml')

# ...