DLL load failed when importing PyQt5
It is because of missing Python3.dll (stub dll, that re-exports Python3x.dll functions, so that one version of extension can work for multiple versions of python).
If your Python distro doesn't bundle python3.dll, you can try one from WinPython (https://winpython.github.io/).
At least the 2017/04/01 versions should have it.
1) Download WinPython ('Zero' version suffices; must be same "main version" - 3.5/3.6 -and "bitness" - 32/64 - as your Python !!!).
2) Extract to some temp directory, take the python3.dll and stick it into your python dir, next to the python3x.dll.
3) Enjoy working QT
I know the topic is old but I have also had this problem with the newest version of PyQT 5.11 but I've downgraded it to 5.9 via:
pip install PyQT5==5.9and it has solved the issue.
If you created a virtualenv, check if python3.dll was copied into the Scripts directory of that virtualenv. Chances are only python35.dll (or python36.dll, etc., depending on the Python version) was copied, in which case you would get the error you're getting.