This package currently uses pkg_resources module from setuptools package:

from pkg_resources import get_distribution

__version__ = get_distribution("m2r2").version

This is not part of the standard library and thus must be declared in setup.py.
As an example, if you are using the Poetry build system, you will not have setuptools by default.
In this case, your doc build will crash in a way that is in my experience hard to understand for lots of users.
Recommended to set lower bounds for the package only, if at all to avoid breaking other people's stuff.