pep8 whole codebase

@jimfulton @strichter
this is just a pep8 pull request. i often do pep8 before moving on to other things...

all tests pass with the current tox config.

there are only 2 pep8 errors left:
~~a `import builtins`, which use i don't know at the moment: https://github.com/zopefoundation/zope.schema/blob/pep8/src/zope/schema/_compat.py#L15~~ removed `import builtins` without any side effects, referred to tests
too long doctest lines at:
https://github.com/zopefoundation/zope.schema/blob/pep8/src/zope/schema/_field.py#L469
and
https://github.com/zopefoundation/zope.schema/blob/pep8/src/zope/schema/_field.py#L479