chore: update PyHEP conveners

Let me know if any corrections are necessary, @eduardo-rodrigues, @oshadura, @matthewfeickert, @nikoladze, and @graeme-a-stewart. Thanks!

_workinggroups/pyhep.md

@@ -11,9 +11,9 @@ the sharing of knowledge and expertise. It embraces the broad community, from HE
 # Conveners
 
 * Eduardo Rodrigues (LHCb, University of Liverpool)
-* Oksana Shadura (IRIS-HEP, University of Nebraska)
 * Jim Pivarski (CMS and IRIS-HEP, Princeton)
 * Matthew Feickert (ATLAS and IRIS-HEP, University of Wisconsin-Madison)
+* Nikolai Hartmann (Belle II, LMU München)
 
 All coordinators can be reached at <[email protected]>.