Use pycodestyle instead of PEP8

PEP8 was renamed to pycodestyle; this issue explains why:
https://github.com/PyCQA/pycodestyle/issues/466

This commit changes our tests to use pycodestyle instead of pep8.

It also means:
- making a couple of whitespace changes to appease the linter
- disabling warnings for bare `Except`s (ie `Except` instead of `Except
  ValueError`) – this seems like a sensible thing to catch but I’m not
  going to make meaningful code changes in this commit

requirements_for_test.txt

@@ -1,5 +1,5 @@
 -r requirements.txt
-pep8==1.7.0
+pycodestyle==2.3.1
 pytest==3.0.1
 pytest-mock==1.2
 pytest-cov==2.3.1