removed pep8 from test dependencies

[corinnebosley]

Closes #905

[pp-mo]

Still to-do here : now stickler is working, remove the whole testclass cartopy.tests.test_coding_standards.TestCodeFormat, as that is all now done in CI not tests.

If I have it right, the idea is that now this is in CI not the test suite. You can perform a manual check with the "flake8" command, which has its own controls in "setup.cfg".

[stickler-ci]

I couldn't find a .stickler.yml file in this repository. I can make one for you, or you can create one by following the documentation.

[corinnebosley]

forgot to rebase...

[corinnebosley]

@pp-mo Could do with your eyes on this one now too please.

[pp-mo]

All looking fine,
except your test_coding_standards is still importing pep8, which needs to go.

If we had stickler working, it would warn about that !
(still waiting on #970)

[corinnebosley]

Oops, quite right @pp-mo, I've pulled it out now.

[pp-mo]

Also checked locally in merge with #970.
All good !