DEPR: DateOffset args don't end in 's'
Make deprecation DateOffset args don't end in 's'.

Mentioned [here](https://github.com/pandas-dev/pandas/pull/36516#discussion_r492885786)

Test1: 

```
pandas/tests/io/generate_legacy_storage_files.py:300: in create_pickle_data
    data = create_data()
pandas/tests/io/generate_legacy_storage_files.py:262: in create_data
    "DateOffset_h_ns": DateOffset(hour=6, nanoseconds=5824),
pandas/_libs/tslibs/offsets.pyx:976: in pandas._libs.tslibs.offsets.RelativeDeltaOffset.__init__
    off, use_rd = _determine_offset(kwds)
```

Test2:

```
test_dt64arr_add_sub_DateOffsets from pandas.tests.arithmetic.test_datetime64.TestDatetime64DateOffsetArithmetic
```

Test3:

```
pandas/tests/tseries/offsets/test_offsets.py:4196: in _test_all_offsets
    self._test_offset(offset_name=name, offset_n=n, **kwds)
pandas/tests/tseries/offsets/test_offsets.py:4199: in _test_offset
    offset = DateOffset(**{offset_name: offset_n})
pandas/_libs/tslibs/offsets.pyx:976: in pandas._libs.tslibs.offsets.RelativeDeltaOffset.__init__
    off, use_rd = _determine_offset(kwds)
```

Test4 (kwd on argument doesn't end with 's'):

```
    @pytest.mark.parametrize("kwd", sorted(liboffsets._relativedelta_kwds))
    def test_valid_relativedelta_kwargs(kwd):
         # Check that all the arguments specified in liboffsets._relativedelta_kwds
         # are in fact valid relativedelta keyword args
         DateOffset(**{kwd: 1})
```

I think these are test relating to this. But I'm not very familiar to handle them. @arw2019  This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @GYAHHA sorry for not responding here. Can you rebase again and will take another look at test failures @GYHHAHA reviving this. I think we should do it. likely need to put `assert_produces_warning(FutureWarning)` around any related tests to get this to pass. move the note to 1.3 ok, I will take this. This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but it appears to have gotten stale. Going to close for now but let us know if you can merge master and update and we'd be happy to reopen.