This rips the actual request flow logic out of website.py and moves it
to flow/request.py, with a linear flow loop remaining in website. I
modified the test client to handle the changes as well and wrote a
single test for now. I also checked it TTW.

Move pytest fixture into the aspen library

Why make that backwards incompatible?

Fix test_flow tests
Nuke fsfix

Update website_flow test
Fix flow tests
Fix socket tests by moving the socket fixtures onto harness
Fix test_resources

The socket testing machinery had a make_request helper, but I want to
use that name for a test helper for test_request.

Fix test_negotiated_resource
Rename want_short_circuit to short_circuit

Want to be able to use this from things besides pytest fixtures.

Clean up remaining import errors in test suite

Fix test_dispatcher
Fix test_negotiated_resource tests to use harness

Fix test_dispatcher tests
Fix some json_resource tests

Fix json_resource test
Fix test_dispatcher tests
Fix configuration tests

Fix up httpbasic and httpdigest tests

With a module docstring to explain it.

Push the setting of `exc_info` and `state` down into flow, and require
the rest to get populated from the return value of flow functions.

These really want to be logged somehow, but our logging is not awesome.

To be parallel to flows.server, which I'm about to add.

This removes the old hooks for website.start/stop.

Now you can do website.server_flow.insert_before('start', func), etc.

I factored out the flow library into a standalone module. This commit
upgrades Aspen to use the standalone version.