Show signature for binding=False cython functions

[user202729]

Fixes #26254 , without binding=True. (That is, this shows the signature correctly in function? for certain functions compiled in Cython.)

Before:

sage: prime_pi??
Type:           PrimePi
String form:    prime_pi
File:           /usr/lib/python3.13/site-packages/sage/functions/prime_pi.pyx
Source:        
cdef class PrimePi(BuiltinFunction):
[...]

After:

sage: prime_pi??
Signature:      prime_pi()
Call signature: prime_pi(self, coerce=True, hold=False, *args)
Type:           PrimePi
String form:    prime_pi
File:           ~/sage/src/sage/functions/prime_pi.pyx
Source:        
cdef class PrimePi(BuiltinFunction):

Even if we ultimately decide to set binding=True, this would probably still be desirable, in case some other library chooses to use binding=False, embedsignature=True.

Note: there's an alternative of implementing this into IPython, see ipython/ipython#14639

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[user202729]

examples mostly copied from sage_getargspec, modified accordingly.

[github-actions]

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[tobiasdiez]

Note: there's an alternative of implementing this into IPython

Would this be much work? Since this seems to be 'just' a cython-specific problem, and nothing intrinsically related to sage, I would prefer if this could be fixed upstream in IPython (in fact, I hope that most of the special Cython handling in sage.inspect could be moved to upstream, either IPython or Python)

[user202729]

Might be worth trying, let's see.