Change all references to the pep8 project to say pycodestyle

[ArgentFalcon]

This fixes issue #518, pep8 still referenced in the cli help command
As a side effect, in setup.cfg now becomes
Also, changed the path for the config file from ~/.config/pep8 to ~/.config/pycodestyle
These feel like changes that should have come with the jump to version 2.0.0, as they are breaking,
but support for as a name can still be added if it's desired enough

[ArcTanSusan]

There's an incomplete sentence in commit message: As a side effect, in setup.cfg now becomes

[ArcTanSusan]

is this PR intended to cover every mention of pep8 in all docs, code, tests? https://gist.github.com/c1b47cf96a899786c3b0b92501d79297 is the output of "git grep pep8" on your branch.

[sigmavirus24]

How does this differ from #519 and #535?

[ArgentFalcon]

@sigmavirus24 It is a similar change to #519 (which I didn't notice until after I made the PR).
The only difference is that I fixed all the references to [pep8] in the tests, so it passes the tests
I also fixed the references in the sphinx docs.

#535 happened after me, so I couldn't give you a rundown of the differences at the moment

@ArcTanSusan it does not change every name reference.
The changes.txt is a record of previous changes, so I felt it was inappropriate all references there
Some references to pep8 are references to the original document, which are correct (both in comments and in the --show-pep8 command, which shows the relevant line from the original pep8 document)
The last one is the pep8codestyle variable in unit tests, which I did not change because of the obvious
change (to pycodestyle) would conflict with the package name, and I did not feel like changing all the
references at the time. If anyone feels like it is necessary, I will go make the necessary changes

As for the remark about the incomplete sentence in the commit message, apparently github stripped the words I put in braces (which make sense because that's a markdown thing). I should have fixed the message not

[xiaopeng163]

your PR is better than mine, I will close #519 , thanks

[IanLee1521]

Looks great, thanks @iamaspacecow ! Also thanks to @xiaopeng163 and @ArcTanSusan for your feedback and input.

Closes #535