The script I used for the bisect was:

---

TARGET_ENV=bisect_env


rm -r ~/.pybuild/$TARGET_ENV || true
git clean -xfd
./configure --prefix=/home/tcaswell/.pybuild/$TARGET_ENV
make -j 9
make install
~/.pybuild/$TARGET_ENV/bin/python3 -m venv --copies --clear ~/.virtualenvs/$TARGET_ENV

popd

source ~/.virtualenvs/$TARGET_ENV/bin/activate
pip install --upgrade pip
echo $PATH

pushd ../sip    # this was at "tip" in the hg repo
pip install .
pip install pyqt5
popd

python -c "import PyQt5.QtCore; PyQt5.QtCore.Qt.Key_Control"
retVal=$?
if [ $retVal -ne 0 ]; then
    exit 1
fi

---


I'm on a relatively up-to-date Arch 

➤  gcc --version
gcc (Arch Linux 9.3.0-1) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


---

updating pip is a key step (that I thoughtlessly carried over from my normal build process) because that lets it see manylinux2014 wheels and does not try to build pyqt5 from source.

python3.9 -m pip freeze
packaging==20.3
pyparsing==2.4.7
PyQt5==5.14.2
PyQt5-sip==12.7.2
sip==5.2.0
six==1.14.0
toml==0.10.0

➤  python3.9 -VV
Python 3.9.0a6+ (heads/master-7-g1c2fa78156:1c2fa78156, May 10 2020, 11:11:03) 
[GCC 9.3.0]


----

I'm also having issues getting pyqt5 to compile from the tarball, what do you mean by "Ah, if I install Python, I can install PyQt5. I installed PyQt5:"?