rename `pep8` linter to `pycodestyle`

[jeffwidman]

The pep8 tool was renamed to pycodestyle.

Historical background detailed in this ticket (from before I switched from Atom to VSCode): AtomLinter/linter-pycodestyle#55

The python.linting.pep8* options in this linter should probably be renamed to python.linting.pycodestyle* to reduce confusion in the future.

Happy to help update the extension, but I'm not sure of your policy on deprecating/migrating settings to new names...??

[brettcannon]

In this instance we would probably implement the new options and then leave the old ones deprecated for several releases until removing them.

[mizzao]

See also DonJayamanne/pythonVSCode#808