Fix installation on modern Python versions

This package no longer installs as of May 2025 and Python 3.13.2:

```console
$ pip install openapi2jsonschema
...
Collecting pyyaml<6.0,>=5.1 (from openapi2jsonschema)
Downloading PyYAML-5.4.1.tar.gz (175 kB)
Installing build dependencies ... done
Getting requirements to build wheel ... error
error: subprocess-exited-with-error
× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> [65 lines of output]
...
AttributeError: cython_sources
[end of output]

note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.
```

The [workaround](https://stackoverflow.com/a/77491847) is as follows:

```bash
pip install "cython<3.0.0" wheel
pip install setuptools
pip install "pyyaml==5.4.1" --no-build-isolation
pip install openapi2jsonschema
openapi2jsonschema --help
```

Or better yet, use [uv](https://docs.astral.sh/uv/) to sandbox all the preceding in a dedicated venv:

```bash
uv init
uv pip install "cython<3.0.0" wheel
uv pip install setuptools
uv pip install "pyyaml==5.4.1" --no-build-isolation
uv pip install openapi2jsonschema
uv run openapi2jsonschema --help
```