BUG: Fix detection of itk package

It is possible that the `itk` metapackage is not install, but one of the
more specific packages were just installed, e.g. `itk-filtering`. Check
for `itk-core`, which will always be installed and which has the
required functionality, when checking for ITK Python.

Author: thewtex

itkwidgets/integrations/itk.py

@@ -4,7 +4,7 @@
 import importlib_metadata
 HAVE_ITK = False
 try:
-    itk_version = importlib_metadata.version('itk')
+    itk_version = importlib_metadata.version('itk-core')
     if version.parse(itk_version) < version.parse('5.3.0'):
         raise RuntimeError('itk 5.3 or newer is required. `pip install itk>=5.3.0`')
     HAVE_ITK = True