upgrade mypy and pyright. close 8 issues

[Dr-Irv]

• Closes Mypy complains about slicing a DateTimeIndex #397
• Closes Regression: Series.apply is too strict, does not allow to return a DateOffset #454
• Closes Series.loc, Series.iloc, DataFrame.loc and DataFrame.iloc not accepting singular slice arguments for __setitem__() (and possibly __getitem()) #572
• Closes Using Timestamp as label when indexing with [] gives typing error. #620
• Closes Make it possible for type checking to detect "The truth value of a DataFrame is ambiguous." #663
• Closes "pathlib.Path / pd.Series" should be inferred as Series, not as Path #682
• Closes CLEAN: Investigate whether any TypeVar that constraints to a list of types can change to being bound to a Union #719
• Closes pd.Series Iterable vs. Sequence #812
• Tests added
  • test_series.py:test_slice_timestamp()
  • test_series.py:test_apply_dateoffset()
  • test_series..py:test_series_single_slice()
  • test_frame.py:test_frame_single_slice()
  • test_series.py:test_series_index_timestamp()
  • test_frame.py:test_frame_index_timestamp()
  • test_series.py:test_series_bool_fails()
  • test_frame.py:test_frame_bool_fails()
  • test_series.py:test_path_div()
  • test_series.py:test_series_dict()
• Updates mypy to 1.14.1 and pyright to 1.391.0
• Moves some series based tests from test_frame.py to test_series.py

[twoertwein]

A few tiny comments. Thank you for fixing soo many issues!

[twoertwein]

Thank you very much @Dr-Irv !