I can reproduce this bug only in Koji, and in Mock if the buildroot is cleaned immediately before. I don't know how to reproduce it on a system/chroot I can SSH into.
Under certain circumstances, when LC_NUMERIC is fr_FR.ISO8859-1 but LC_ALL is C.UTF-8, locale.localeconv() fails with
UnicodeDecodeError: 'locale' codec can't decode byte 0xa0 in position 0: Invalid or incomplete multibyte or wide character
Apparently, the thousands separator (or something else) in the lconv is "\xa0" (unbreakable space in fr_FR.ISO8859-1), and it's being decoded with UTF-8.
This is tripped by Python's test suite, namely test_float.GeneralFloatCases.test_float_with_comma
One failed Koji build is here: https://koji.fedoraproject.org/koji/taskinfo?taskID=21419062
A smaller reproducer is here (self-contained SPEC file): https://gist.github.com/encukou/70b3d3f9ef3e29ac1dbc23a5f7bd6431
I was able to reproduce this with the self-contained SPEC file on rawhide, but not on f27 or f26.
rawhide: https://koji.fedoraproject.org/koji/taskinfo?taskID=21425550 (failed)
f27: https://koji.fedoraproject.org/koji/taskinfo?taskID=21425571 (completed)
f26: https://koji.fedoraproject.org/koji/taskinfo?taskID=21425582 (completed)
I bet the change in glibc is relevant.
I can verify that the issue is with the latest glibc and more specifically with the 2.26.90 version.
My tests compared version 2.26-15.fc27.x86_64 to 2.26.90-3.fc28 .
The easiest way to reproduce it, is to initialize a F27 chroot or VM, run the test (it will pass on this step) and then upgrade the five glibc* packages that come with the base install to those from  and rerun the test (it will fail here).
dnf will complain during upgrading so it's better to wget the rpm's and then install them through 'rpm -Uvh <rpm's>'.
A PR has been submitted upstream which fixes the issue . Waiting on it to be merged before backporting it.
python3-3.6.4-6.fc27 has been submitted as an update to Fedora 27. https://bodhi.fedoraproject.org/updates/FEDORA-2018-ff1b193de3
python3-3.6.4-6.fc27 has been pushed to the Fedora 27 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2018-ff1b193de3
python3-3.6.4-6.fc27 has been pushed to the Fedora 27 stable repository. If problems still persist, please make note of it in this bug report.