ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@1b16b76599)
  (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/2254a7e/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 2021-08-17
-  set(ITK_GIT_TAG "3a63e79d17")
+  # ITK nightly-master 2021-08-18
+  set(ITK_GIT_TAG "1b16b76599")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '5.2.1.dev20210817+250.g3a63e79d17'
+VERSION = '5.2.1.dev20210818+251.g1b16b76599'
 
 def get_versions():
     """Returns versions for the ITK Python package.