Qcore

* Start publishing manylinux wheels
* Support Python 3.7
* Improve pickle implementation for enums
* Add type stubs

* Improve installation procedure

* Add qcore.caching.lru_cache
* Declare Cython as a setup_requires dependency

* Add support for some new features in py3 (e.g. keyword-only args).
* Make pickling/unpickling of enums work across py2 and py3.
* Make helpers.object_from_string handle more cases.

* Various enum improvements. Invalid values are now disallowed, methods like
get_names() return members in consistent order, there is a new IntEnum
class, and some new methods were added.
* Cython is no longer required to import qcore if it has not been compiled with
Cython.
* qcore.caching.memoize() is now more efficient.

* MarkerObject names are now always text
* Fix equality for DecoratorBinder objects to work correctly in compiled and
non-compiled Python 2 and 3.
* Errors run during event handlers are now reraised with a meaningful
traceback.