Change "pls" to "please" in warning message
 - [ ] closes #xxxx
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry

Code is now more formal when talking to users, as to not disrespect them!
Full warning:
`"by argument to sort_index is deprecated, please use .sort_values(by=...)"`
Hello @alanbato! Thanks for submitting the PR.

 - In the file [`pandas/core/frame.py`](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py), following are the PEP8 issues :

> [Line 2045:9](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py#L2045): [E722](https://duckduckgo.com/?q=pep8%20E722) do not use bare except'
> [Line 2479:13](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py#L2479): [E722](https://duckduckgo.com/?q=pep8%20E722) do not use bare except'
> [Line 3362:80](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py#L3362): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (80 > 79 characters)
> [Line 5221:21](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py#L5221): [E722](https://duckduckgo.com/?q=pep8%20E722) do not use bare except'
> [Line 5717:9](https://github.com/pandas-dev/pandas/blob/f263535633cc952838ff44f28ca9a6aee2c87fb7/pandas/core/frame.py#L5717): [E722](https://duckduckgo.com/?q=pep8%20E722) do not use bare except'

 thanks!