'FigureManager' is an undefined name in backend_wx.py

[cclauss]

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)
                 ^

[tacaswell]

These are injected via the

matplotlib/lib/matplotlib/backends/backend_wx.py

Line 1969 in 5d30d6a

@_Backend.export

class decorator at the bottom.