mpc() constructor should be compatible with complex()

Instead, now if the ``real`` argument is a complex number - the second (``imag``) argument just ignored:
```pycon
>>> complex(real:=3+1j, imag:=7-1j)
(4+8j)
>>> real + imag*1j
(4+8j)
>>> mpmath.mpc(real, imag)
mpc(real='3.0', imag='1.0')
```