Modernize and simplify iris.analysis._Groupby (SciTools#5015)

* Modernize and simplify _Groupby

* Rename variable to improve readability

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

* Add a whatsnew entry

* Add a type hint to _add_shared_coord

* Add a test for iris.analysis._Groupby.__repr__

---------

Co-authored-by: Martin Yeo <40734014+trexfeathers@users.noreply.github.com>

Author: 2 people

docs/src/whatsnew/latest.rst

@@ -144,6 +144,9 @@ This document explains the changes made to Iris for this release
 💼 Internal
 ===========
 
+#. `@bouweandela`_ and `@trexfeathers`_ (reviewer) modernized and simplified
+   the code of ``iris.analysis._Groupby``. (:pull:`5015`)
+
 #. `@fnattino`_ changed the order of ``ncgen`` arguments in the command to
    create NetCDF files for testing  (caused errors on OS X). (:pull:`5105`)