* prepare for python3.6 support and migrate to pipenv

* merge with master

* refactor code style and build targets

* improved docs and more cleanup

* more documentation

* revert behavior of normalize_headers

* comment out passthrough tests for now

* update tox

* include singledispatch dependency

* ci

* skip lock

* fix testserver

* pyopenssl

* Ignore IDE metadata

* Python 2.6 is EOL and was dropped in 2015

* Use automatic formatters

Callbacks are now called when the response
is about to be written.
A callback now takes the actual request as
first parameter. And can be any object that is
callable. Furthermore the signature of the
return value has changed to a tuple returning
the status code along with the headers and the body.
This allows for more logic in the callbacks.

Callbacks and regex url matching now work together
because the real request is used for passing the
url the callback instead of the Regex