DEPR: Add deprecation warning for factorize() order keyword
State: closed

#### What's this PR do?
In response to #19727.
Adds functionality to the ``deprecate_kwargs`` utility.  Currently, the utility doesn't allow the deprecation of an old keyword without having a new one in place.  The change proposed here allows the deprecation of an *old* keyword without specifying a *new* keyword by allowing users to pass ``new_arg_name=None``  to ``deprecate_kwargs``. 

#### PR checklist
- [X] closes #19727
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry


Labels:
name: Deprecate
description: Functionality to remove in pandas

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@695614d`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19751/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19751   +/-   ##
=========================================
  Coverage          ?   91.58%           
=========================================
  Files             ?      150           
  Lines             ?    48894           
  Branches          ?        0           
=========================================
  Hits              ?    44781           
  Misses            ?     4113           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <100%> (?)` | |
| #single | `41.8% <42.85%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/19751/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.16% <100%> (ø)` | |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/19751/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `82.4% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=footer). Last update [695614d...b7ed3c4](https://codecov.io/gh/pandas-dev/pandas/pull/19751?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

Hello @EricChea! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 21, 2018 at 11:37 Hours UTC
can you add a whatsnew note in the deprecation section, otherwise lgtm.
thanks @EricChea 

small correction on merge, the note was in the incorrect deprecation section.
@jreback thanks for doing that.