test: prepare test/pseudo-tty/testcfg.py for Python 3

reduce() was moved into functools in Python 3

• https://docs.python.org/3/library/functools.html#functools.reduce

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines