Try to get Windows CI working again

When I added the hack to get Python 3.6 running on our AppVeyor Windows CI,
everything passed, but now we have a problem. The images generated on
Windows/Py36 are now different from what we've historically expected. This
isn't shocking since my hack was to fix the version of pyqt to avoid a library
incompatibility issue, for Python 3.6 only.

Let's try applying the same version constraint *everywhere*. From my
experiments, this seems to make Windows/Py3.x create images agreeing with the
'webkit' test corpus, while Windows/Py2.7 still agrees with 'webkit_win'. One
clear difference between the two is the reported OpenGL renderer, so revise
the test logic to factor that in when choosing which corpus to compare
against.

appveyor.yml

@@ -13,15 +13,14 @@ environment:
                         # to the matrix section.
 
       CONDA_CHANNELS: "astrofrog/label/dev conda-forge"
-      CONDA_DEPENDENCIES: "astropy qtpy traitlets ipywidgets>=7.0 ipyevents widgetsnbextension pyqt pytest pytest-cov>=2.6.1 pytest-remotedata>=0.3.1 requests matplotlib flask flask-cors pyopengl"
+      # Without the pyqt version constraint, a lib compat issue leads to import failures on qtpy for Python 3.6:
+      CONDA_DEPENDENCIES: "astropy qtpy traitlets ipywidgets>=7.0 ipyevents widgetsnbextension pyqt==5.6.0 pytest pytest-cov>=2.6.1 pytest-remotedata>=0.3.1 requests matplotlib flask flask-cors pyopengl"
       PIP_DEPENDENCIES: "pytest-faulthandler codecov reproject"
 
   matrix:
       - PYTHON_VERSION: "2.7"
       - PYTHON_VERSION: "3.5"
       - PYTHON_VERSION: "3.6"
-        # Without the pyqt version constraint, a lib compat issue leads to import failures on qtpy:
-        CONDA_DEPENDENCIES: "astropy qtpy traitlets ipywidgets>=7.0 ipyevents widgetsnbextension pyqt==5.6.0 pytest pytest-cov>=2.6.1 pytest-remotedata>=0.3.1 requests matplotlib flask flask-cors pyopengl"
 
 # matrix:
 #     fast_finish: true

pywwt/conftest.py

@@ -40,7 +40,10 @@ if QT_INSTALLED and OPENGL_INSTALLED:
             return info
 
 
+_cached_opengl_renderer = ''
+
 def pytest_report_header(config):
+    global _cached_opengl_renderer
 
     lines = []
 
@@ -69,6 +72,10 @@ def pytest_report_header(config):
             lines.append("OpenGL Version: {0}".format(opengl_info['version']))
             lines.append("Shader Version: {0}".format(opengl_info['shader']))
 
+            # This is (no surprise) a hack to enable the Windows testing
+            # framework to check which renderer WebKit is using, which affets
+            # the output.
+            _cached_opengl_renderer = opengl_info['renderer']
         else:
 
             lines.append("Could not determine OpenGL version (OpenGL package required)")

pywwt/tests/test_qt_widget.py

@@ -79,8 +79,11 @@ with open('actual.png', 'wb') as f:
 def assert_widget_image(tmpdir, widget, filename):
     actual = tmpdir.join(filename).strpath
     widget.render(actual)
+
+    from ..conftest import _cached_opengl_renderer
+
     framework = 'webengine' if WEBENGINE else 'webkit'
-    if sys.platform.startswith('win') and not WEBENGINE:
+    if sys.platform.startswith('win') and not WEBENGINE and 'GDI' in _cached_opengl_renderer:
         framework += '_win'
     elif sys.platform.startswith('darwin'):
         framework += '_osx'