Don't docstest `extern` contents

[nstarman]

The extern module contains bundled codes. As the default, doctstring-plus tests may/should not be applied ?

I have a branch open with this small change. Should I PR?

in tox.ini

[tool:pytest]
doctest_norecursedirs=*/{{ cookiecutter.module_name }}/extern/*

[pllim]

https://github.com/astropy/package-template#deprecation-warning