Backport PR
#9952: Some mpl backends have no corresponding GUI
Hopefully fixes a test failure on Windows where mpl tries to activate an `agg` gui.
Should we manually define the backend2gui dict instead of inverting backends and then modifying it?
Test failure:
```
Traceback (most recent call last):
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\unittest\case.py", line 58, in testPartExecutor
yield
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\unittest\case.py", line 600, in run
testMethod()
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\nose\failure.py", line 39, in runTest
raise self.exc_val.with_traceback(self.tb)
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\nose\loader.py", line 418, in loadTestsFromName
addr.filename, addr.module)
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\nose\importer.py", line 47, in importFromPath
return self.importFromDir(dir_path, fqname)
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\nose\importer.py", line 94, in importFromDir
mod = load_module(part_fqname, fh, filename, desc)
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\imp.py", line 234, in load_module
return load_source(name, filename, file)
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\imp.py", line 172, in load_source
module = _load(spec)
File "<frozen importlib._bootstrap>", line 693, in _load
File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 665, in exec_module
File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
File "c:\jenkins\workspace\windows-multi\_pyversion\3.5\label\windows\IPython\core\tests\test_pylabtools.py", line 18, in <module>
from matplotlib import pyplot as plt
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\matplotlib\pyplot.py", line 2512, in <module>
install_repl_displayhook()
File "C:\Users\ADMINI~1\AppData\Local\Temp\2\1845\lib\site-packages\matplotlib\pyplot.py", line 165, in install_repl_displayhook
ip.enable_gui(ipython_gui_name)
File "c:\jenkins\workspace\windows-multi\_pyversion\3.5\label\windows\IPython\terminal\interactiveshell.py", line 463, in enable_gui
self._inputhook = get_inputhook_func(gui)
File "c:\jenkins\workspace\windows-multi\_pyversion\3.5\label\windows\IPython\terminal\pt_inputhooks\__init__.py", line 38, in get_inputhook_func
raise UnknownBackend(gui)
IPython.terminal.pt_inputhooks.UnknownBackend: No event loop integration for 'agg'. Supported event loops are: qt, qt4, qt5, gtk, gtk2, gtk3, tk, wx, pyglet, glut, osx
```
Signed-off-by: Thomas Kluyver <thomas@kluyver.me.uk>
