Fix tests with IPython 9

[olebole]

From @chronitis https://bugs.debian.org/1129367:

Autopkgtest test_invocation:test_invoke_command_ipython fails with IPython 9.x (having passed with 8.x).

It looks like this is due to the pyraf test suite assuming that the output from IPython can be decoded as ASCII-only; I'm not sure exactly what it chokes on, but presumably it now emits some unicode in a standard session:

pyraf/pyraf/tests/test_invocation.py

Line 69 in 1e75e49

self.stdout = self.stdout.decode('ascii')

I think this is a test-only issue, although it's possible a similar runtime issue might be buried in pyraf/ipython_api.py