Repair memslice test after disabling auto-detection of read-only views, but keep some of the tests for 'const' views as smoke tests.

Disable auto-detection of read-only memory views (too unsafe) and acquire them only as read-only if the memory view dtype is a "const" type.

See #2102.

Specify license in cython-mode.el's header

Rewrite documentation paragraph on automatic string encoding for clarity

Disallow memory views as struct members since structs do not allow safe buffer reference management.

Use NamedTemporaryFile instead of mkstemp

Move user branch hint generation from IfClauseNode into IfStatNode to include also the inverse 'else' condition.

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.