Inject branch hints for user conditionals that raise exceptions as we can expect them to be unlikely.

Closes #2096.

Closes #2097.

Disable the specialised "tp_getattro" implementations in Py3.7 as setting the slot to anything but "PyObject_GenericGetAttr" disables the much more effective method call optimisation that avoids creating an intermediate PyMethod object in Py3.7.

Use a dedicated ExprNode class for Pythran implemented NumPy methods to move them out of the way of Python call optimisations.

Add some Py3-only builtin names (mostly I/O exceptions) as uncachable builtins to avoid "unknown name" errors when running Cython in Py2.
Closes #2087.

Add sigaltstack()

Promote the new opaque PEP-539 type "Py_tss_t" to a known Cython type to avoid static variable initialisation issues with the "Py_tss_NEEDS_INIT" struct initialiser value.

Pass "never None" property of decoded byte strings through to the code that uses them.

Use the same function return type inference for builtin types in GeneralCallNode as in SimpleCallNode.

Fix enum-to-unicode conversion code in C++ mode where direct assignments from C int to enum are not allowed.

Also reduces the C code overhead by removing the explicit 1-arg tuple packing code.

Disallow old-style classes for multiple inheritance

Use a specialised GenericGetAttr implementation for the "tp_getattro" slot whenever a type uses the generic lookup but has no instance dict.

Remove the complete copy of "PyObject_GenericGetAttr()" again as it's too much code for too little effect. Calling the original C-API function as a fallback should be enough.

Optimise attribute access on extension types with "__getattr__" but without instance dict through a streamlined copy of "PyObject_GenericGetAttr".

Optimise attribute access on Cython provided extension types without instance dict through a streamlined copy of "PyObject_GenericGetAttr".

Add warnings to compiler directive docs

Use specialised default return handling code for "return None" in generators and other functions that return Python objects.

Fix link to Pythran manual in the documentation.

C functions without args should be declared as f(void)