fix: resolve mypy 2.3 errors in strings.py and test_datasource.py

- strings.py: cast tostring() return value to T since mypy 2.3 no longer
  accepts bytes as compatible with TypeVar T constrained to str|bytes
- test_datasource.py: add explicit `import unittest.mock` since mypy 2.3
  no longer resolves submodule attributes through the parent import

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacalata

tableauserverclient/helpers/strings.py

@@ -1,6 +1,6 @@
 from defusedxml.ElementTree import fromstring, tostring
 from functools import singledispatch
-from typing import TypeVar, overload
+from typing import TypeVar, cast, overload
 
 # the redact method can handle either strings or bytes, but it can't mix them.
 # Generic type so we can write the actual logic once, then use singledispatch to
@@ -17,8 +17,9 @@ def _redact_any_type(xml: T, sensitive_word: T, replacement: T, encoding=None) -
         matches = root.findall(".//password")
         for item in matches:
             item.text = "********"
-        # tostring returns bytes unless an encoding value is passed
-        return tostring(root, encoding=encoding)
+        # tostring returns bytes unless an encoding value is passed; cast since
+        # the callers pass encoding="unicode" for str and None for bytes
+        return cast(T, tostring(root, encoding=encoding))
     except Exception:
         # something about the xml handling failed. Just cut off the text at the first occurrence of "password"
         location = xml.find(sensitive_word)

test/test_datasource.py

@@ -3,6 +3,7 @@
 from pathlib import Path
 import tempfile
 import unittest
+import unittest.mock
 from zipfile import ZipFile
 
 from defusedxml.ElementTree import fromstring