Check for Qt version instead of python version where appropriate

Previously there was an implicit assumption that Python2 comes with Qt4,
while Python 3 comes with Qt5. This is not necessarily true.
Now we explicitly check the Qt version when we need different code for
different Qt versions

ert_gui/main_window.py

@@ -1,11 +1,11 @@
 import functools
 import os
 import pkg_resources
-import sys
 import webbrowser
 import yaml
 
 from ErtQt.Qt import QSettings, Qt, QMainWindow, qApp, QWidget, QVBoxLayout, QDockWidget, QAction, QToolButton
+from ErtQt import QT5
 
 from ert_gui.about_dialog import AboutDialog
 
@@ -105,14 +105,13 @@ def closeEvent(self, event):
 
 
     def __fetchSettings(self):
-        py3 = sys.version_info[0] == 3
         settings = QSettings("Equinor", "Ert-Gui")
         geo = settings.value("geometry")
         if geo:
-            self.restoreGeometry(geo if py3 else geo.toByteArray())
+            self.restoreGeometry(geo if QT5 else geo.toByteArray())
         wnd = settings.value("windowState")
         if wnd:
-            self.restoreState   (wnd if py3 else wnd.toByteArray())
+            self.restoreState   (wnd if QT5 else wnd.toByteArray())
 
 
     def setWidget(self, widget):

ert_gui/tools/plot/style_chooser.py

@@ -1,9 +1,6 @@
 import sys
 
-try:
-  from PyQt4.QtGui import QWidget, QHBoxLayout, QComboBox, QDoubleSpinBox, QLabel, QHBoxLayout
-except ImportError:
-  from PyQt5.QtWidgets import QWidget, QHBoxLayout, QComboBox, QDoubleSpinBox, QLabel, QHBoxLayout
+from ErtQt.Qt import QWidget, QHBoxLayout, QComboBox, QDoubleSpinBox, QLabel, QHBoxLayout
 
 from ert_gui.plottery import PlotStyle
 
@@ -136,7 +133,8 @@ def _updateStyle(self):
         thickness = float(self.thickness_spinner.value())
         size = float(self.size_spinner.value())
 
-        if sys.version_info[0] == 2:
+        from ErtQt import QT4
+        if QT4:
             self._style.line_style = str(line_style.toString())
             self._style.marker = str(marker_style.toString())
         else:

ert_gui/tools/plot/widgets/clearable_line_edit.py

@@ -1,13 +1,13 @@
 import sys
 
-try:
-  from PyQt4.QtCore import QString, QSize, Qt
-  from PyQt4.QtGui import QPushButton, QColor, QLineEdit, QStyle
-except ImportError:
-  from PyQt5.QtCore import QSize, Qt
-  from PyQt5.QtWidgets import QPushButton, QLineEdit, QStyle
-  from PyQt5.QtGui import QColor
-
+from ErtQt.Qt import (
+    QColor,
+    QLineEdit,
+    QPushButton,
+    QSize,
+    QStyle,
+    Qt,
+    )
 
 from ert_gui.ertwidgets import resourceIcon
 
@@ -100,7 +100,9 @@ def setText(self, string):
 
     def text(self):
         if self._placeholder_active:
-            if sys.version_info[0] == 2:
+            from ErtQt import QT4
+            if QT4:
+                from ErtQt.Qt import QString
                 return QString("")
             else:
                 return ""