pytest: UsageError in package that contains pytest_plugins module

When running pytest --doctest-modules on a package that contains a pytest plugin, pytest errors out and produces a pretty long stacktrace. The code that triggers the error is the _get_plugin_specs_as_list function that receives a module object instead of a list or a string.

The behavior is OS-independent as far as I can tell, but affects only pytest 3.7+ ; pytest 3.6.4 works fine. Also note that the error is only triggered when doc-testing the specific subpackage that contains the plugin.

For proper context, we have the following directory structure:

odl
 |
 |- util --
 |        |- pytest_plugins.py
 |        |- ...
 | - ...

The error is triggered by pytest --doctest-modules odl/util.

The full backtrace is here. Link to the subpackage: https://github.com/odlgroup/odl/tree/master/odl/util

Edit: Updated pytest versions affected