setStepType only available in Qt 5.12
Hi,

I got this exception with `silx view`, packaged for debian10 (Qt5 version 5.11.3):
```
  File "/usr/lib/python3/dist-packages/silx/gui/dialog/ColormapDialog.py", line 869, in __init__
    self._gammaSpinBox.setStepType(qt.QDoubleSpinBox.AdaptiveDecimalStepType)
```

This function was introduced in Qt 5.12.

This is a regression introduced recently by:
https://github.com/silx-kit/silx/pull/3054


diff --git a/silx/gui/dialog/ColormapDialog.py b/silx/gui/dialog/ColormapDialog.py
index 4cee10536..707e9e6b7 100644
--- a/silx/gui/dialog/ColormapDialog.py
+++ b/silx/gui/dialog/ColormapDialog.py
@@ -866,7 +866,7 @@ class ColormapDialog(qt.QDialog):
         self._gammaSpinBox = qt.QDoubleSpinBox(parent=self)
         self._gammaSpinBox.setEnabled(False)
         self._gammaSpinBox.setRange(0., 1000.)
-        self._gammaSpinBox.setStepType(qt.QDoubleSpinBox.AdaptiveDecimalStepType)
+        self._gammaSpinBox.setSingleStep(0.1)
         self._gammaSpinBox.valueChanged.connect(self._gammaUpdated)
         self._gammaSpinBox.setValue(2.)
 
['silx/gui/dialog/ColormapDialog.py']