RecursionError in Dependency Resolution with "sphinx-rtd-theme" in requirements.txt

## Description
I am experiencing a `RecursionError` when using `python-inspector` to analyze a project that includes `sphinx-rtd-theme` as a dependency. This error seems to occur during the dependency resolution process.

### Steps to Reproduce

1. Create a `requirements.txt` file with the following content:

```
sphinx-rtd-theme
```
2. Run `python-inspector` with the following command:
```
python-inspector --python-version 311 --operating-system macos --json-pdt output.json --analyze-setup-py-insecurely --requirement requirements.txt --verbose
```

## Expected Behavior
I expected `python-inspector` to analyze the project's dependencies without any issues.

## Actual Behavior
The `python-inspector` command results in a `RecursionError`. The detailed error output is as follows:

```
dependencies = [pdt_dfs(mapping, graph, c) for c in children]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/inderpal.singh/.pyenv/versions/3.11.5/lib/python3.11/site-packages/python_inspector/resolution.py", line 739, in
dependencies = [pdt_dfs(mapping, graph, c) for c in children]
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/inderpal.singh/.pyenv/versions/3.11.5/lib/python3.11/site-packages/python_inspector/resolution.py", line 736, in pdt_dfs
key=src, package_name=src, installed_version=str(mapping[src].version), dependencies=[]
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/inderpal.singh/.pyenv/versions/3.11.5/lib/python3.11/site-packages/packvers/version.py", line 301, in __str__
parts.append(".".join(str(x) for x in self.release))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RecursionError: maximum recursion depth exceeded
```