Istio importer fails

Hi Team,

I have noticed this error when I tried to start the Istio importer:

```
./manage.py import vulnerabilities.importers.istio.IstioImporter
Importing data using vulnerabilities.importers.istio.IstioImporter
Traceback (most recent call last):
File "/app/vulnerabilities/management/commands/import.py", line 70, in import_data
ImportRunner(importer).run()
File "/app/vulnerabilities/import_runner.py", line 62, in run
count = self.process_advisories(advisory_datas=advisory_datas, importer_name=importer_name)
File "/app/vulnerabilities/import_runner.py", line 105, in process_advisories
for data in advisory_datas:
File "/app/vulnerabilities/importers/istio.py", line 59, in advisory_data
yield from self.process_file(file=file, base_path=base_path)
File "/app/vulnerabilities/importers/istio.py", line 65, in process_file
advisory_url = get_advisory_url(
File "/app/vulnerabilities/utils.py", line 533, in get_advisory_url
relative_path = str(file.relative_to(base_path)).strip("/")
AttributeError: 'str' object has no attribute 'relative_to'
Failed to run importer vulnerabilities.importers.istio.IstioImporter. Continuing...
CommandError: 1 failed!: vulnerabilities.importers.istio.IstioImporter
```