This is not an eric issue but caused by the operating system. There are two possibilities 
I am aware of.

1. There seems to be a Windows configuration to disable the path length limit of 256 
chracters. Unfortunately I don't know which one it is.
2. Create a Python virtual environment (e.g. C:\Users\David\eric_env) and install eic in 
there. This has the advantage, that you can have different versions of Python and/or the 
packages (like PyQt5) on your system. Details about this may be found on the eric web 
site.

When trying to install Eric-ide on a windows 10 achine the installtion fails from pip and install.py downloads. 
Tracing the error appears to be with filename length. 

The error message provided is 

ERROR: Could not install packages due to an EnvironmentError: [Errno 2] No such file or directory: 'C:\\Users\\David\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\\LocalCache\\local-packages\\Python38\\site-packages\\eric6\\Documentation\\Source\\eric6.Plugins.WizardPlugins.QRegularExpressionWizard.QRegularExpressionWizardCharactersDialog.html'

checking the relavent directory finds the file
eric6.Plugins.WizardPlugins.QRegExpWizard.QRegExpWizardCharactersDialog.html

I cant rename it as the filename is too long for windows.

The pip install manifest needs to replect the correct filename but I dont know  how to change it.