In the case where alternate separator is None, we use '' which will
always be in any string. We want to skip that case.

Also we only run our tests on AppVeyor, not all of our testenvs.

When normalizing paths, we want to handle the following cases:

- Someone is using a Windows-style path on Windows
- Someone is using a Unix style path on Unix
- Someone is using a Unix style path on Windows

os.path.sep will handle the native directory separator character while
os.path.altsep (when set) will handle alternate separators. Further,
os.path.abspath does the right thing on Windows when handed a Unix-style
path.

Related to #175

This simplifies the changes, reduces the scope of refactors apparently
for refactoring's sake and ensures that the internals are reasonable.

It also airs on the side of preserving information rather than
discarding or overwriting it.

For the sake of IDEs, check filename for exclusion even if the file is directly
named in the command line.

Also, if the filename is "-" (stdin) check the provided display name for
exclusion.

Also, avoid calling path checking functions on the "-" filename:

 * fnmatch.fnmatch()
 * os.path.isdir()
 * os.path.exists()

On Flake8 2.x we added the information about the implementation,
version, and operating system to the --version output to make helping
users easier. In short they can pretty simply just give us the output
from

    flake8 --version

And we can get a lot of the information that we need.

Previously, pycodestyle never introspected the argument names for
classes except to require that ``tree`` be an argument it could pass.
For Flake8 3.0, we lifted that restriction, but old plugins seem to
have cargo-culted their __init__ signature to be

    def __init__(self, tree, builtins=None):

For some yet unknown reason. This was causing an AttributeError. By
updating flake8.utils.parameters_for to return a dictionary that
indicates whether the parameter is required or not, we can side-step
this by simply ignoring the parameter if it has a default value and
we cannot provide it.

Closes #151

This fixes a regression in behaviour from 2.x to 3.

Closes #150

This is an emerging best practice and there is little reason to not
follow it