Fix failing viscm editor test

The old behavior was an implicit conversion to `int`. This warning was
being thrown in Python 3.9:

```
DeprecationWarning: an integer is required (got type float).  Implicit conversion to integers using __int__ is deprecated, and may be removed in a future version of Python.
```

And in Python 3.10, this turned in to an error, as seen in
<#71>:

```
TypeError: setValue(self, a0: int): argument 1 has unexpected type 'float'
```

The new version of this code simply makes the old behavior (conversion
to int) explicit. I'm unsure whether this was the original intent.

Author: mfisher87

.github/workflows/test.yml

@@ -42,3 +42,7 @@ jobs:
 
       - name: "Run tests"
         run: "make test"
+        env:
+          # In Pythons >= 3.10, tests fail with `RuntimeError: Invalid DISPLAY
+          # variable`, unless this variable is set:
+          MPLBACKEND: "Agg"

test/test_gui.py

@@ -1,7 +1,5 @@
 from pathlib import Path
 
-import pytest
-
 from viscm.cli import _make_window
 
 
@@ -21,9 +19,6 @@ def test_gui_view_opens(self, qtbot):
 
         assert window.isVisible()
 
-    @pytest.mark.xfail(
-        reason="Unknown. See https://github.com/matplotlib/viscm/issues/71",
-    )
     def test_gui_edit_pyfile_opens(self, tests_data_dir: Path, qtbot):
         """Reproduce viridis from README instructions.
 

viscm/gui.py

@@ -1154,7 +1154,7 @@ def __init__(self, figurecanvas, viscm_editor, parent=None):
         self.max_slider = QtWidgets.QSlider(QtCore.Qt.Horizontal)
         self.max_slider.setMinimum(0)
         self.max_slider.setMaximum(100)
-        self.max_slider.setValue(viscm_editor.max_Jp)
+        self.max_slider.setValue(int(viscm_editor.max_Jp))
         self.max_slider.setTickPosition(QtWidgets.QSlider.TicksBelow)
         self.max_slider.setTickInterval(10)
         self.max_slider.valueChanged.connect(self.updatejp)
@@ -1164,7 +1164,7 @@ def __init__(self, figurecanvas, viscm_editor, parent=None):
         self.min_slider = QtWidgets.QSlider(QtCore.Qt.Horizontal)
         self.min_slider.setMinimum(0)
         self.min_slider.setMaximum(100)
-        self.min_slider.setValue(viscm_editor.min_Jp)
+        self.min_slider.setValue(int(viscm_editor.min_Jp))
         self.min_slider.setTickPosition(QtWidgets.QSlider.TicksBelow)
         self.min_slider.setTickInterval(10)
         self.min_slider.valueChanged.connect(self.updatejp)
@@ -1277,8 +1277,8 @@ def update_smoothness_slider(self):
 
     def swapjp(self):
         jp1, jp2 = self.min_slider.value(), self.max_slider.value()
-        self.min_slider.setValue(jp2)
-        self.max_slider.setValue(jp1)
+        self.min_slider.setValue(int(jp2))
+        self.max_slider.setValue(int(jp1))
         self.updatejp()
 
     def updatejp(self):