Fix issue#1712: Update `BaseQAWithSourcesChain` to handle space & newline after `SOURCES:` (#2118)

Fix the issue outlined in #1712 to ensure the `BaseQAWithSourcesChain`
can properly separate the sources from an agent response even when they
are delineated by a newline.

This will ensure the `BaseQAWithSourcesChain` can reliably handle both
of these agent outputs:

* `"This Agreement is governed by English law.\nSOURCES: 28-pl"` ->
`"This Agreement is governed by English law.\n`, `"28-pl"`
* `"This Agreement is governed by English law.\nSOURCES:\n28-pl"` ->
`"This Agreement is governed by English law.\n`, `"28-pl"`

I couldn't find any unit tests for this but please let me know if you'd
like me to add any test coverage.

langchain/chains/qa_with_sources/base.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import re
 from abc import ABC, abstractmethod
 from typing import Any, Dict, List, Optional
 
@@ -116,8 +117,8 @@ class BaseQAWithSourcesChain(Chain, BaseModel, ABC):
     def _call(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
         docs = self._get_docs(inputs)
         answer, _ = self.combine_documents_chain.combine_docs(docs, **inputs)
-        if "SOURCES: " in answer:
-            answer, sources = answer.split("SOURCES: ")
+        if re.search(r"SOURCES:\s", answer):
+            answer, sources = re.split(r"SOURCES:\s", answer)
         else:
             sources = ""
         result: Dict[str, Any] = {