feat(pip-compile): Treat .txt files as pip_requirements files

This fixes a case I missed in renovatebot#28959 where a lock file uses another lock
file as an input file.  In that case, we need to treat the input lock
file as a pip_requirements file so that we extract the
--[extra-]index-url flags from it and pass the correct credentials to
pip-compile.

lib/modules/manager/pip-compile/common.spec.ts

@@ -342,5 +342,10 @@ describe('modules/manager/pip-compile/common', () => {
       expect(matchManager('file.in')).toBe('pip_requirements');
       expect(matchManager('another_file.in')).toBe('pip_requirements');
     });
+
+    it('matches pip_requirements any .txt file', () => {
+      expect(matchManager('file.txt')).toBe('pip_requirements');
+      expect(matchManager('another_file.txt')).toBe('pip_requirements');
+    });
   });
 });

lib/modules/manager/pip-compile/common.ts

@@ -320,7 +320,7 @@ export function matchManager(filename: string): SupportedManagers | 'unknown' {
     return 'pep621';
   }
   // naive, could be improved, maybe use pip_requirements.fileMatch
-  if (filename.endsWith('.in')) {
+  if (filename.endsWith('.in') || filename.endsWith('.txt')) {
     return 'pip_requirements';
   }
   return 'unknown';