upgrade mypy and pyright. close 4 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
• 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()
• 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

[Dr-Irv]

closing in favor of #1084