flake8 testing of https://github.com/matplotlib/matplotlib on Python 3.6.4
$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
./lib/matplotlib/backends/backend_wx.py:1485:18: F821 undefined name 'FigureManager'
figmgr = FigureManager(canvas, 1, self)
^
./lib/matplotlib/backends/backend_wx.py:1548:18: F821 undefined name 'FigureManager'
figmgr = FigureManager(canvas, 1, frame)
^
flake8 testing of https://github.com/matplotlib/matplotlib on Python 3.6.4
$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics