CPython math.nan is positive with regards to copysign. The signaling bit
(aka sign flag) was incorrectly set.
In addition, REPR_C and REPR_D should only use the _true_ nan to prevent
system crash in case of hand-crafted floats. For instance, with REPR_C,
any nan-like float following the pattern
`01111111 1xxxxxxx xxxxxxxx xxxxx1xx` would be switched to an immediate
object or a qstr string. When the qstr index is too large, this would
cause a crash.
This commit fixes the issue, and adds the relevant test cases.
Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>
