There is a module like this:

"""I am a module.

Examples
--------
>>> import yaml  # optional dependency
>>> do_stuff
"""

__doctest_requires__ = {'*': ['yaml']}

# Real code to follow

In a CircleCI job with pytest-xdist installed and parallel run is enabled, the doctest fails with ImportError: import yaml failed but the other jobs passed.

xref astropy/astropy#10257