Maint: cts: refrain from async (await) in Python code for being reserved

... as of Python 3.7:
https://docs.python.org/3/whatsnew/3.7.html#summary-release-highlights

One of the instance attributes turned out to be bogus.

Randomly spotted; unfortunately byte compilation when building package
for Fedora will not consider unability to pregenerate bytecode a fatal
problem.