I have little idea about the environment you are working in, what you have tried or the reason for the error. Looking on the internet I found several possible solutions.
Take a look at C:\Windows\System32 and verify the names of the dll's you have installed. If they are different, change the name and that's it. The dilemma is to know which dll is giving the error.
Have updated pip, so the solution is to update pip and reinstall pyqt5.
python -m pip install --upgrade pip
python -m pip install pyqt5
Reference here and also here y here
I am not sure about this solution because it is for PyQt4 but you could try.
Reinstall Python in administrator mode for all users of the pc, selecting "add to PATH" in the first step. Open the command prompt in administrator mode and update pip using the command
python -m pip install mitmproxy (which I took from this linl because when trying to run
pip install --upgrade pip was giving an access denied error. After that, download PyQt4 as .whl from this link making sure it was the version for Python 3.5. Finally, install the .whl with pip install "file".whl from the folder that contains it.
If none of this works, someone more knowledgeable will have to come along to help you;)