inline: Fix a ridiculously subtle flexattr bug

As detailed here:
#2406 (comment)

In a *function-style* definition, we didn't properly *un-define* the
values for a given item after each function invocation. So when a field
wasn't defined, it would get the value for the previously-formatted
object instead. It now properly throws a NameError.

beetsplug/inline.py

@@ -117,9 +117,13 @@ def _expr_func(obj):
             # For function bodies, invoke the function with values as global
             # variables.
             def _func_func(obj):
+                old_globals = dict(func.__globals__)
                 func.__globals__.update(_dict_for(obj))
                 try:
                     return func()
                 except Exception as exc:
                     raise InlineError(python_code, exc)
+                finally:
+                    func.__globals__.clear()
+                    func.__globals__.update(old_globals)
             return _func_func

docs/changelog.rst

@@ -12,6 +12,15 @@ New features:
   Thanks to :user:`dosoe`.
   :bug:`2580` :bug:`3272`
 
+Fixes:
+
+* :doc:`/plugins/inline`: In function-style field definitions that refer to
+  flexible attributes, values could stick around from one function invocation
+  to the next. This meant that, when displaying a list of objects, later
+  objects could seem to reuse values from earlier objects when they were
+  missing a value for a given field. These values are now properly undefined.
+  :bug:`2406`
+
 For plugin developers:
 
 * `MediaFile`_ has been split into a standalone project. Where you used to do