fix: invalid escape sequence error in regex pattern (#3902)

This PR fixes the "SyntaxError: invalid escape sequence" error in the
pydantic.py file. The issue was caused by the backslashes in the regular
expression pattern being treated as escape characters. By using a raw
string literal for the regex pattern (e.g., r"\{.*\}"), this fix ensures
that backslashes are treated as literal characters, thus preventing the
error.

Co-authored-by: Tomer Levy <tomer.levy@tipalti.com>

langchain/output_parsers/pydantic.py

@@ -17,7 +17,7 @@ class PydanticOutputParser(BaseOutputParser[T]):
         try:
             # Greedy search for 1st json candidate.
             match = re.search(
-                "\{.*\}", text.strip(), re.MULTILINE | re.IGNORECASE | re.DOTALL
+                r"\{.*\}", text.strip(), re.MULTILINE | re.IGNORECASE | re.DOTALL
             )
             json_str = ""
             if match: