BUG: Roundtrip with openpyxl and datetime precision 
The test `tests.io.excel.test_writers.test_datetimes` fails with openpyxl (now the default, xref #35029):

```
path = "test.xlsx"
datetimes = [
    datetime(2013, 1, 13, 1, 2, 3),
    datetime(2013, 1, 13, 2, 45, 56),
    datetime(2013, 1, 13, 4, 29, 49),  # <---
    datetime(2013, 1, 13, 6, 13, 42),
    datetime(2013, 1, 13, 7, 57, 35),  # <---
    datetime(2013, 1, 13, 9, 41, 28),
    datetime(2013, 1, 13, 11, 25, 21),
    datetime(2013, 1, 13, 13, 9, 14),
    datetime(2013, 1, 13, 14, 53, 7),
    datetime(2013, 1, 13, 16, 37, 0),
    datetime(2013, 1, 13, 18, 20, 52),
]
write_frame = DataFrame({"A": datetimes})
write_frame.to_excel(path, "Sheet1")
read_frame = pd.read_excel(path, sheet_name="Sheet1", header=0, engine="openpyxl")
tm.assert_series_equal(write_frame["A"], read_frame["A"])
```

The highlighted lines give 

     2013-01-13 04:29:48.999999
     2013-01-13 07:57:35.000001

With these two lines removed, the results are of the form e.g. `2013-01-13 11:25:21` and the test passes.