Tests: Adjust expected exception message for Python 3.14.0a6 (#3500)

E       AssertionError: Regex pattern did not match.
    E        Regex: '3 cannot cast to typing.Union\\[str, int\\]'
    E        Input: '3 cannot cast to str | int'

Change caused likely by python/cpython@dc6d66f

Author: hroncok

docs/changelog/3500.bugfix.rst

@@ -0,0 +1,2 @@
+Make tox tests pass with Python 3.14.0a6
+- by :user:`hroncok`

tests/config/loader/test_str_convert.py

@@ -77,7 +77,7 @@ def test_str_convert_ok_py39(raw: str, value: Any, of_type: type[Any]) -> None:
     [
         ("a", TypeVar, TypeError, r"a cannot cast to .*typing.TypeVar.*"),
         ("3", Literal["1", "2"], ValueError, r"3 must be one of \('1', '2'\)"),
-        ("3", Union[str, int], TypeError, r"3 cannot cast to typing.Union\[str, int\]"),
+        ("3", Union[str, int], TypeError, r"3 cannot cast to (typing.Union\[str, int\]|str \| int)"),
         ("", Command, ValueError, r"attempting to parse '' into a command failed"),
     ],
 )