Replace `pep8` package with `pycodestyle`

[addianto]

The app produces warning message when analysing a project. The warning message said that pep8 package has been renamed to pycodestyle, as suggested in https://github.com/PyCQA/pycodestyle/issues/466. The warning message is as follows:

[shortened for brevity]
app_1  | 172.23.0.1 - - [31/Dec/2019 08:31:13] "POST /check HTTP/1.1" 302 -
app_1  | Cloning into 'pyreportcard'...
app_1  | mypy: can't read file 'tmp/pyreportcard/config.py': No such file or directory
app_1  | /usr/local/lib/python3.6/site-packages/pep8.py:2124: UserWarning:
app_1  |
app_1  | pep8 has been renamed to pycodestyle (GitHub issue #466)
app_1  | Use of the pep8 tool will be removed in a future release.
app_1  | Please install and use `pycodestyle` instead.

[mingrammer]

Yap. I was aware of this issue. I'll fix it in the near future.

Check this please: https://github.com/mingrammer/pyreportcard/issues/2#issuecomment-569952033