Bug report

Bug summary
Can't use qt4agg, error reported below.

Downgrade to matplotlib 3.1.3 solves the problem.

Code for reproduction

import matplotlib as mpl
mpl.use('qt4agg')

Or, set it to Qt4Agg in matplotlibrc file.

Actual outcome

ImportError: cannot import name '_isdeleted' 
Failed to enable GUI event loop integration for 'qt5'

Matplotlib version

• Operating system: win 10 LTSC 2019
• Matplotlib version: 3.2.1
• Matplotlib backend (print(matplotlib.get_backend())): can only use TkAgg
• Python version: 3.6