Add Cython as dependency

It seems cython is required in certain circumstances, see error below. Please add cython to `requirements.txt`.

```
08:50:03.364 [DefaultDispatcher-worker-1] ERROR org.ossreviewtoolkit.plugins.packagemanagers.python.Pip - Unable to determine dependencies for definition file '/builds/oss/oss-review-toolkit/ort-gitlab-ci/project/requirements.txt': IOException: Running 'python-inspector --python-version 310 --operating-system linux --json-pdt /tmp/ort-PythonInspector16980114111782037340/python-inspector8413559668320372377.json --analyze-setup-py-insecurely --requirement /builds/oss/oss-review-toolkit/ort-gitlab-ci/project/requirements.txt --verbose' in '/builds/oss/oss-review-toolkit/ort-gitlab-ci/project' failed with exit code 1:
Traceback (most recent call last):
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolve_cli.py", line 247, in resolve_dependencies
resolution_result: Dict = resolver_api(
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/api.py", line 254, in resolve_dependencies
resolution, purls = resolve(
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/api.py", line 313, in resolve
resolved_dependencies, packages = get_resolved_dependencies(
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/api.py", line 351, in get_resolved_dependencies
resolver_results = resolver.resolve(requirements=requirements, max_rounds=max_rounds)
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/resolvelib/resolvers.py", line 546, in resolve
state = resolution.resolve(requirements, max_rounds=max_rounds)
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/resolvelib/resolvers.py", line 427, in resolve
failure_causes = self._attempt_to_pin_criterion(name)
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/resolvelib/resolvers.py", line 239, in _attempt_to_pin_criterion
criteria = self._get_updated_criteria(candidate)
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/resolvelib/resolvers.py", line 229, in _get_updated_criteria
for requirement in self._p.get_dependencies(candidate=candidate):
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 639, in get_dependencies
return list(self._iter_dependencies(candidate))
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 630, in _iter_dependencies
for r in self.get_requirements_for_package(purl=purl, candidate=candidate):
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 509, in get_requirements_for_package_from_pypi_simple
yield from get_requirements_from_python_manifest(
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 299, in get_requirements_from_python_manifest
yield from get_reqs_insecurely(
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 277, in get_reqs_insecurely
yield from parse_reqs_from_setup_py_insecurely(setup_py=setup_py_location)
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/resolution.py", line 128, in parse_reqs_from_setup_py_insecurely
for req in iter_requirements(level="", extras=[], setup_file=setup_py):
File "/opt/python/versions/3.9.5/lib/python3.9/site-packages/python_inspector/setup_py_live_eval.py", line 117, in iter_requirements
exec(file_contents, g)
File "", line 793, in
File "", line 763, in setup_package
File "", line 520, in maybe_cythonize
RuntimeError: Cannot cythonize without Cython installed.
```