Update documentation links.

Added a link to readable documentation, viewable from GitHub.

Fix possibility of rare crash during dealloc. Fixes #66

remove build files in manylinux builds

CI setup for manylinux wheels on tagged commit

Avoid raising a SystemError when clearing slots if setstate() failed.

PR #52 introduced a code path to `ghostify` that calls PyErr_Clear()
with the intent to avoid propagating AttributeErrors for slots.

However, if there is an error (like a POSKeyError) raised by
jar.setstate(), then `unghostify` will call ghostify with an error
pending. If the object had slots that weren't set and the
AttributeError was cleared, so was the pending error from setstate. So
when `ghostify` returned NULL that got propagated up to the
interpreter which finds no exception and so raises `SystemError: error
return without exception set`.

This commit makes `unghostify` save and restore the exception state
around the call to PyErr_Clear.

Fix ``__setstate__`` interning when ``state`` parameter is not a built-in dict

Use macpython for all Mac builds.

Ref #58

Python 3.6 Mac

Support Python 3.6