Implement PEP 515: allow underscores in number literals

Remove obsolete forward about (extremely) old versions of Cython.

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with MSVC.

The current order results in compiler warnings on 32 bit machines, e.g.

src/llfuse.c: In function '__Pyx_PyInt_TrueDivideObjC':
src/llfuse.c:43980:17: warning: left shift count >= width of type
                 if (8 * sizeof(long) <= 53 || (__Pyx_sst_abs(size) <= 52 / PyLong_SHIFT) || likely(labs(a) <= (1L << 53))) {

Switching the order so that the left shift is closer to the sizeof test
avoids the warning, presumably because it makes it easier for the
compiler to see that the left shift is only executed on 64 bit.

Thanks to Christian Neukirchen for doing most of the work!

The current cast isn't doing anything, because the unary minus implicitly
upcasts to int again, resulting in warnings about potentially value altering
conversions on 32 bit systems.

Fixes issue #877.

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	Cython/Compiler/ExprNodes.py
	Cython/Shadow.py

unpack FormattedValueNode with constant string value at compile time (unlikely case but trivial to do...)

set FormattedValueNode.format_spec field to None for empty format specs to simplify later "empty" tests