Add PyBytes_FromObject() and PyObject_Bytes()

Closes GH-2936.

Closes GH-2939.

Adapt PyCode_New() calls to changed C-API in Py3.8, which now takes an additional argument for the pos-only args count.
Closes GH-2938.

Keep METH_O optimisation when "always_allow_keywords" is enabled but the function has only one pos-only argument.

Adds positional only args support (PEP 570)

GH-2919: Prevent tuple constants that compare equal but have different item types from being merged into one.

GH-2934: improve error reporting for non-constant exception value expressions

GH-2934: improve error reporting for non-constant exception value expressions: instead of two errors and a crash, just report it once and explicitly.

Clean weakrefs before calling user's dealloc

Add PyGILState_Check to Includes/cpython/pystate.pxd

Closes #2926.

We must clean the weakreferences before calling the user's __dealloc__
because if the __dealloc__ releases the GIL, a weakref can be
dereferenced accessing the object in an inconsistent state or
resurrecting it.

The comments were taken from the CPython docs.

Closes #2713.