Flaky builds around PyQt5/PySide pip downloads

[altendky]

pip and PyPI are having a hard time together on PyQt5 and PySide2 wheels. The packages download incompletely and then fail hash checks. 1) Why are the downloads stopping short? 2) Why is pip failing that out on the hash instead of the incomplete download?

https://github.com/twisted/qt5reactor/runs/934639858?check_suite_focus=true
https://github.com/twisted/qt5reactor/runs/930744031?check_suite_focus=true

ERROR: THESE PACKAGES DO NOT MATCH THE HASHES FROM THE REQUIREMENTS FILE. If you have updated the package versions, please update the hashes. Otherwise, examine the package contents carefully; someone may have tampered with them.
    pyside2==5.15.0 from https://files.pythonhosted.org/packages/24/39/348e72c1607ac12915e1809e9092e09aa239bc48ace41d51253179e635bb/PySide2-5.15.0-5.15.0-cp35.cp36.cp37.cp38-abi3-manylinux1_x86_64.whl#sha256=f9099e49fb2d3571f5a81eb9ff281ce832ce8c333052e8175e2356b9c3e4a882:
        Expected sha256 f9099e49fb2d3571f5a81eb9ff281ce832ce8c333052e8175e2356b9c3e4a882
             Got        43a032f3b7b72f1e0139ac9c4d798ec4a947a7343f014bb70714f266b5d75a21

Reported:

• PyQt5 and PySide2 wheels sometimes fail hash checks after incomplete download pypi/warehouse#8330

Other resolutions:

• pip harder altendky/qtrio#124

[altendky]

Since we are using tox here to install the pure bash retry workaround in QTrio won't directly apply.