[roottest] Inherit python executable from main process in nbdiff.py

This makes sure that we're using the exact Python version in the
notebook tests that ROOT was built against.

Author: guitargeek

roottest/python/JupyROOT/nbdiff.py

@@ -17,7 +17,7 @@
     "%s "
     "--output %s"
 )
-pythonInterpName = "python3"
+pythonInterpName = sys.executable
 
 rootKernelFileContent = (
     """{
@@ -161,16 +161,16 @@ def getKernelName(inNBName):
     with open(inNBName) as f:
         nbj = json.load(f)
     if nbj["metadata"]["kernelspec"]["language"] == "python":
-        return pythonInterpName
+        return "python3"
     else:  # we support only Python and C++
         return "root"
 
 
-def canReproduceNotebook(inNBName, kernelName, needsCompare):
+def canReproduceNotebook(inNBName, needsCompare):
     tmpDir = addEtcToEnvironment(os.path.dirname(inNBName))
     outNBName = inNBName.replace(nbExtension, "_out" + nbExtension)
     interpName = getInterpreterName()
-    convCmd = convCmdTmpl % (interpName, kernelName, inNBName, outNBName)
+    convCmd = convCmdTmpl % (interpName, getKernelName(inNBName), inNBName, outNBName)
     exitStatus = os.system(convCmd)  # we use system to inherit the environment in os.environ
     shutil.rmtree(tmpDir)
     if needsCompare:
@@ -206,7 +206,5 @@ def isInputNotebookFileName(filename):
     except:
         raise ImportError("Cannot import jupyter")
 
-    kernelName = getKernelName(nbFileName)
-
-    retCode = canReproduceNotebook(nbFileName, kernelName, needsCompare)
+    retCode = canReproduceNotebook(nbFileName, needsCompare)
     sys.exit(retCode)