RCFNum constructor fails with _get_ctx error

[mikesol]

Hi all,

First, thanks so much for your hard work on this amazing tool!

I wanted to try z3rcf but ran into the following issue:

>>> from z3 import *
>>> z3rcf.RCFNum(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\MikeSolomon\devel\wall\src\.venv\lib\site-packages\z3\z3rcf.py",
line 60, in __init__
    self.ctx = z3._get_ctx(ctx)
AttributeError: module 'z3' has no attribute '_get_ctx'

Am I doing something wrong here? If this is a bug, I'm happy to dig and propose a PR, but I'm not sure if there's a mistake in my code.

[NikolajBjorner]

fixed