worker: adapt _blockingreader to work around a python3.8.[0-1] bug (issue6444)

Python 3.8.0 is the latest I can load on Ubuntu 18.04, and I regularly hit the TypeError because this function is missing. While it can be avoided by disabling worker usage via config option, that's a bit obscure.

I'm limiting the function definition to the narrow range of affected pythons because there were other bugs in this area that were worked around, that I don't fully understand. See the bug report for discussions on why the narrow range, and related commits working around other bugs.

Differential Revision: https://phab.mercurial-scm.org/D12627