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
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