feat(pip_requirements): Extract flags from package files with no deps

lib/modules/manager/pip_requirements/extract.spec.ts

@@ -239,5 +239,43 @@ some-package==0.3.1`;
         ],
       });
     });
+
+    it('extracts a file with only --index-url flags', () => {
+      const res = extractPackageFile('--index-url https://example.com/pypi');
+      expect(res).toMatchObject({
+        deps: [],
+        registryUrls: ['https://example.com/pypi'],
+      });
+    });
+
+    it('extracts a file with only --extra-index-url flags', () => {
+      const res = extractPackageFile(
+        '--extra-index-url https://example.com/pypi',
+      );
+      expect(res).toMatchObject({
+        deps: [],
+        additionalRegistryUrls: ['https://example.com/pypi'],
+      });
+    });
+
+    it('extracts a file with only -r flags', () => {
+      const res = extractPackageFile('-r requirements-other.txt');
+      expect(res).toMatchObject({
+        deps: [],
+        managerData: {
+          requirementsFiles: ['requirements-other.txt'],
+        },
+      });
+    });
+
+    it('extracts a file with only -c flags', () => {
+      const res = extractPackageFile('-c constraints.txt');
+      expect(res).toMatchObject({
+        deps: [],
+        managerData: {
+          constraintsFiles: ['constraints.txt'],
+        },
+      });
+    });
   });
 });

lib/modules/manager/pip_requirements/extract.ts

@@ -131,7 +131,13 @@ export function extractPackageFile(
       return dep;
     })
     .filter(is.truthy);
-  if (!deps.length) {
+  if (
+    !deps.length &&
+    !registryUrls.length &&
+    !additionalRegistryUrls.length &&
+    !additionalRequirementsFiles.length &&
+    !additionalConstraintsFiles.length
+  ) {
     return null;
   }
   const res: PackageFileContent = { deps };