ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@d98f9612c6)
  (2) VERSION variable set in itkVersion.py

It was automatically generated by the script update_python_version.sh [1]

[1] https://github.com/InsightSoftwareConsortium/ITKPythonPackage/blob/8e70aef/scripts/update_python_version.py

Author: kwrobot

CMakeLists.txt

@@ -74,8 +74,8 @@ if(ITKPythonPackage_SUPERBUILD)
 
   set(ITK_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
 
-  # ITK nightly-master 2021-08-24
-  set(ITK_GIT_TAG "f87c9bdaee")
+  # ITK nightly-master 2021-08-31
+  set(ITK_GIT_TAG "d98f9612c6")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '5.2.1.dev20210824+254.gf87c9bdaee'
+VERSION = '5.2.1.dev20210831+256.gd98f9612c6'
 
 def get_versions():
     """Returns versions for the ITK Python package.