DOC: Fix flake8 issues in doc/source/whatsnew/v0.23.0.rst

[saurav2608]

We didn't start validating the format of PEP8 and other code standards in the documentation examples until recently. We still have some files with errors, that we need to skip, and that we should fix, so we can start validating them.

The first step of this issue would be edit setup.cfg in the pandas home, and in the flake8-rst section, remove from the exclude list the file doc/source/whatsnew/v0.23.0.rst

After that, running the next command will report the errors in the file (note that syntax error usually prevent to validate other errors, and the list of errors to fix can become much longer when the syntax error is fixed (please make sure that you are using flake8-rst version 0.7.0 or higher):

$ flake8-rst doc/source/whatsnew/v0.23.0.rst 
source/whatsnew/v0.23.0.rst:72:18: E999 SyntaxError: invalid syntax
source/whatsnew/v0.23.0.rst:339:83: E501 line too long (80 > 79 characters)
source/whatsnew/v0.23.0.rst:699:84: E501 line too long (87 > 79 characters)
source/whatsnew/v0.23.0.rst:828:84: E501 line too long (94 > 79 characters)

Once all the errors are addressed, please open a pull request with the fixes in the file, and removing the file from setup.cfg. If you need to do something that feels wrong to fix an error, please ask in a comment to this issue. Please avoid other unrelated changes, which can be addressed in a separate pull request.
cc: @datapythonista

[WillAyd]

Closed via #24217