fix(manager:pip_requirements): declare git-tags datasource (#15444)

lib/modules/manager/pip_requirements/index.ts

 import { ProgrammingLanguage } from '../../../constants';
+import { GitTagsDatasource } from '../../datasource/git-tags';
 import { PypiDatasource } from '../../datasource/pypi';
 
 export { updateArtifacts } from './artifacts';
@@ -11,4 +12,4 @@ export const defaultConfig = {
   fileMatch: ['(^|/)([\\w-]*)requirements\\.(txt|pip)$'],
 };
 
-export const supportedDatasources = [PypiDatasource.id];
+export const supportedDatasources = [PypiDatasource.id, GitTagsDatasource.id];