Running tests fail

Hi,

Do you happen to know why the following test might fail?

```
tly cleaning up
_warnings.warn(warn_message, ResourceWarning)
test_plugin_on_serve (mkdocs_monorepo_plugin.tests.test_plugin.TestMonorepoPlugin) ... ok
test_plugin_on_serve_no_run (mkdocs_monorepo_plugin.tests.test_plugin.TestMonorepoPlugin) ... ok

======================================================================
ERROR: test_plugin_on_config_with_nav (mkdocs_monorepo_plugin.tests.test_plugin.TestMonorepoPlugin)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tmp/guix-build-python-mkdocs-monorepo-plugin-1.0.4.drv-0/mkdocs-monorepo-plugin-1.0.4/mkdocs_monorepo_plugin/tests/test_plugin.py", line 25, in test_plugin_on_config_with_nav
plugin.on_config({
File "/tmp/guix-build-python-mkdocs-monorepo-plugin-1.0.4.drv-0/mkdocs-monorepo-plugin-1.0.4/mkdocs_monorepo_plugin/plugin.py", line 52, in on_config
new_docs_dir = self.merger.merge()
File "/tmp/guix-build-python-mkdocs-monorepo-plugin-1.0.4.drv-0/mkdocs-monorepo-plugin-1.0.4/mkdocs_monorepo_plugin/merger.py", line 74, in merge
raise SystemExit(1)
SystemExit: 1

----------------------------------------------------------------------
Ran 4 tests in 0.003s

```

Start with mkdocs_monorepo_plugin/tests/test_plugin.py at test_plugin_on_config_with_nav and reproduce the failure. Trace the call through mkdocs_monorepo_plugin/plugin.py line 52 to mkdocs_monorepo_plugin/merger.py line 74, then determine why merge exits; done means the test completes without SystemExit and the full test run passes.

Written by the indexing model from the issue text.